CM4000: fix broken flash base for OpenGear boards

Use _bss_start_ofs as the size of the boot loader code+data that we want
to protect in the flash. This replaces use of the no longer defined
_armboot_start.

Fixes:

flash.c: In function ‘flash_init’:
flash.c:75: error: ‘_bss_start’ undeclared (first use in this function)
flash.c:75: error: (Each undeclared identifier is reported only once
flash.c:75: error: for each function it appears in.)
flash.c:75: error: ‘_armboot_start’ undeclared (first use in this function)

Signed-off-by:  <greg.ungerer@opengear.com>