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>

board/cm4008/flash.c

@@ -72,7 +72,7 @@ unsigned long flash_init (void)
 	 */
 	flash_protect (FLAG_PROTECT_SET,
 		       CONFIG_SYS_FLASH_BASE,
-		       CONFIG_SYS_FLASH_BASE + _bss_start - _armboot_start,
+		       CONFIG_SYS_FLASH_BASE + _bss_start_ofs,
 		       &flash_info[0]);
 
 	return size;

board/cm41xx/flash.c

@@ -72,7 +72,7 @@ unsigned long flash_init (void)
 	 */
 	flash_protect (FLAG_PROTECT_SET,
 		       CONFIG_SYS_FLASH_BASE,
-		       CONFIG_SYS_FLASH_BASE + _bss_start - _armboot_start,
+		       CONFIG_SYS_FLASH_BASE + _bss_start_ofs,
 		       &flash_info[0]);
 
 	return size;