Kconfig: Use hex values for CONFIG_{SPL,TPL}_SIZE_LIMIT

CONFIG_{SPL,TPL}_SIZE_LIMIT are defined as hex (SPL_SIZE_LIMIT was
converted in b51882d0 ("spl: Convert CONFIG_SPL_SIZE_LIMIT to hex"), but
there are still places that reference integer values. Change those to hex
as well. Also, update the Makefile to check for 0x0 instead of 0.

This also fixes the following build error when CONFIG_SPL_SIZE_LIMIT
is set by menuconfig to 0x0:
...
spl/u-boot-spl.bin exceeds file size limit:
  limit:  0 bytes
  actual: 0x80f0 bytes
  excess: 0x80f0 bytes

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Reviewed-by: Simon Glass <sjg@chromium.org>