microblaze: Avoid compile error on systems without cfi flash

microblaze: Avoid compile error on systems without cfi flash

Use XILINX_FLASH_START to set/unset FLASH and RAMENV.

Error:
board.c: In function 'board_init':
board.c:134: error: 'XILINX_FLASH_START' undeclared (first use in this function)
board.c:134: error: (Each undeclared identifier is reported only once
board.c:134: error: for each function it appears in.)

Signed-off-by: Stephan Linz <linz@li-pro.net>
Signed-off-by: Michal Simek <monstr@monstr.eu>