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.

@@ -18,18 +18,33 @@
 #include <sys/machparam.h>
 #include <sys/cpu_asm.h>
 
 #include "assym.h"
 
-#if defined(__lint)
-
-#endif
+/*
+ * Every story needs a beginning. This is ours.
+ */
 
 /*
  * Each of the different machines has its own locore.s to take care of getting
- * us into fakebop for the first time. After that, they all return here to a
- * generic locore to take us into mlsetup and then to main forever more.
+ * the machine specific setup done.  Just before jumping into fakebop the
+ * first time, we call this machine specific code.
+ */
+
+/*
+ * We are in a primordial world here. The loader is going to come along and
+ * boot us at _start. 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 what the loader set up for us:
+ *
+ * - unaligned access are allowed (A = 0, U = 1)
+ * - virtual memory is enabled
+ *   - we (unix) are mapped right were we want to be
+ *   - a UART has been enabled & any memory mapped registers have been 1:1
+ *     mapped
+ *   - ATAGs have been updated to tell us what the mappings are
+ * - I/D L1 caches have been enabled
  */
 
         /*
          * External globals
          */

@@ -42,10 +57,49 @@
 
         .data
         .comm   t0stack, DEFAULTSTKSZ, 32
         .comm   t0, 4094, 32
 
+
+/*
+ * 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
+
+        /* invoke machine specific setup */
+        bl      _mach_start
+
+        bl      _fakebop_start
+        SET_SIZE(_start)
+
+
 #if defined(__lint)
 
 /* ARGSUSED */
 void
 _locore_start(struct boot_syscalls *sysp, struct bootops *bop)

@@ -140,5 +194,15 @@
         SET_SIZE(_locore_start)
 
 __return_from_main:
         .string "main() returned"
 #endif  /* __lint */
+
+        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)