nxp: Rename CONFIG_U_BOOT_HDR_SIZE to FSL_U_BOOT_HDR_SIZE

This is always defined to 16K, so we move this over to
include/fsl_validate.h to start with. Next, we rename this from CONFIG_
to FSL_. Coalesce the various comments around this definition to be in
fsl_validate.h as well to explain the usage.

Signed-off-by: Tom Rini <trini@konsulko.com>