mcu/pic32: Add minimal boot code

There is special flash area dedicated to bootloader (0xbfc00000-0xbfc0FF00).
Code from this area is execute first.
This is where mcuboot is placed.

It is possible to build application elf that provides minimal
code that jumps to normal flash area dedicated for applications.
The elf generated this way can then easyly be used in MPLAB X ide
instead of command line gdb.

When syscfg has MCU_NO_BOOTLOADER_BUILD set to 1
additional section is generated that goes to bootloader area of MCU.

kernel/os/src/arch/pic32/startup/crt0.S

@@ -633,6 +633,25 @@ __crt0_exit:
         .globl __crt0_exit
         .end _main_entry
 
+#if MYNEWT_VAL_MCU_NO_BOOTLOADER_BUILD
+        ##################################################################
+        # This is startup code for no boot loader application.
+        # This jumps from boot flash to program flash.
+        # It is not used for bootload/application build since it would
+        # generate huge image.
+        ##################################################################
+        .section .boot_reset,code
+        .align 2
+        .set reorder
+        .ent _boot_reset
+        _boot_reset:
+        la      v0, _app_reset
+        jr      v0
+
+        .align 2
+        .end _boot_reset
+#endif
+
         .section .text.SystemInit,code
         .align 2
         .weak SystemInit