[coreboot] buildrom rom size patch

[Myles Watson]

This patch makes use of the CONFIG_COREBOOT_V3_ROM_SIZE variable in buildrom.
The hope is that eventually we could do some math with it if we want.  The
only problem is that when you don't have it set, it gives you an error when
you do make menuconfig.  Here is the error:

#
# using defaults found in .config
#
.config:23:warning: symbol value '' invalid for COREBOOT_V3_ROM_SIZE
.config:83:warning: symbol value '' invalid for LAB_PAUSE

Notice that LAB_PAUSE has the same problem.  Setting the value to be a string
instead of an int removes the warning, but removes the error checking as well.

Myles

Signed-off-by: Myles Watson