ARM: remove unused variable

The "size" variable in start_armboot() in lib_arm/board.c is only really
used in "#ifndef CONFIG_SYS_NO_FLASH" case, and even there it can be
eliminated (thanks to Jean-Christophe PLAGNIOL-VILLARD for a suggestion.)

Signed-off-by: Guennadi Liakhovetski <lg@denx.de>