configure: Properly handle MM_DEBUG

configure: Properly handle MM_DEBUG

Define MM_DEBUG in config.h when --enable-mm-debug is passed to configure.
It was being defined in config-util.h which only gets used when building
GRUB utilities for the host side. The enabling of debugging for memory
management in include/grub/mm.h explicitly does not happen when compiling
for the GRUB utilities. So this debugging code effectively could never be
enabled. Note, that MM_DEBUG is defined in an #if directive because the
enabling of debugging checks if MM_DEBUG is defined, not what its value is.
So even if MM_DEBUG were defined to nothing, the debugging code would
still be enabled.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>