armv6: bcm2835 & qvpb have nearly identical locore _start
It makes sense to common-ize _start for all armv6 machines.  They will all
have to do the same basic setup.  If there is any machine specific setup
they need to do, they can do so in the new _mach_start function.

*** 16,116 ****
  
  #include <sys/asm_linkage.h>
  #include <sys/machparam.h>
  #include <sys/cpu_asm.h>
  
! /*
!  * Every story needs a beginning. This is ours.
!  */
! 
! /*
!  * We are in a primordial world here. The BMC2835 is going to come along and
!  * boot us at _start. Normally we would go ahead and use a main() function, but
!  * for now, we'll do that ourselves. As we've started the world, we also need to
!  * set up a few things about us, for example our stack pointer. To help us out,
!  * it's useful to remember the rough memory map. Remember, this is for physcial
!  * addresses. There is no virtual memory here. These sizes are often manipulated
!  * by the 'configuration' in the bootloader.
!  *
!  * +----------------+ <---- Max physical memory
!  * |                |
!  * |                |
!  * |                |
!  * +----------------+
!  * |                |
!  * |      I/O       |
!  * |  Peripherals   |
!  * |                |
!  * +----------------+ <---- I/O base 0x20000000 (corresponds to 0x7E000000)
!  * |                |
!  * |     Main       |
!  * |    Memory      |
!  * |                |
!  * +----------------+ <---- Top of SDRAM
!  * |                |
!  * |       VC       |
!  * |     SDRAM      |
!  * |                |
!  * +----------------+ <---- Split determined by bootloader config
!  * |                |
!  * |      ARM       |
!  * |     SDRAM      |
!  * |                |
!  * +----------------+ <---- Bottom of physical memory 0x00000000
!  *
!  * With the Raspberry Pi Model B, we have 512 MB of SDRAM. That means we have a
!  * range of addresses from [0, 0x20000000). If we assume that the minimum amount
!  * of DRAM is given to the GPU - 32 MB, that means we really have the following
!  * range: [0, 0x1e000000).
!  *
!  * By default, this binary will be loaded into 0x8000. For now, that means we
!  * will set our initial stack to 0x10000000.
!  */
! 
! /*
!  * Recall that _start is the traditional entry point for an ELF binary.
!  */
!         ENTRY(_start)
!         ldr sp, =t0stack
!         ldr r4, =DEFAULTSTKSZ
!         add sp, r4
!         bic sp, sp, #0xff
! 
!         /*
!          * establish bogus stacks for exceptional CPU states, our exception
!          * code should never make use of these, and we want loud and violent
!          * failure should we accidentally try.
!          */
!         cps #(CPU_MODE_UND)
!         mov sp, #-1
!         cps #(CPU_MODE_ABT)
!         mov sp, #-1
!         cps #(CPU_MODE_FIQ)
!         mov sp, #-1
!         cps #(CPU_MODE_IRQ)
!         mov sp, #-1
!         cps #(CPU_MODE_SVC)
! 
!         /* Enable highvecs (moves the base of the exception vector) */
!         mrc     p15, 0, r3, c1, c0, 0
!         mov     r4, #1
!         lsl     r4, r4, #13
!         orr     r3, r3, r4
!         mcr     p15, 0, r3, c1, c0, 0
! 
          /* Enable access to p10 and p11 (privileged mode only) */
          mrc     p15, 0, r0, c1, c0, 2
          orr     r0, #0x00500000
          mcr     p15, 0, r0, c1, c0, 2
  
!         bl _fakebop_start
!         SET_SIZE(_start)
! 
!         ENTRY(arm_reg_read)
!         ldr r0, [r0]
!         bx lr
!         SET_SIZE(arm_reg_read)
! 
!         ENTRY(arm_reg_write)
!         str r1, [r0]
!         bx lr
!         SET_SIZE(arm_reg_write)
--- 16,28 ----
  
  #include <sys/asm_linkage.h>
  #include <sys/machparam.h>
  #include <sys/cpu_asm.h>
  
!         ENTRY(_mach_start)
          /* Enable access to p10 and p11 (privileged mode only) */
          mrc     p15, 0, r0, c1, c0, 2
          orr     r0, #0x00500000
          mcr     p15, 0, r0, c1, c0, 2
  
!         bx      r14
!         SET_SIZE(_mach_start)