flash: Add optional verify-after-write feature

Sometimes it might make sense to verify the written data to NOR flash.
This patch adds this feature. To enable this verify-after-write, you
need to define CONFIG_FLASH_VERIFY in your board config header.

Please note that this option is useless in nearly all cases,
since such flash programming errors usually are detected earlier
while unprotecting/erasing/programming. Please only enable
this option if you really know what you are doing.

Signed-off-by: Stefan Roese <sr@denx.de>