[memmap] Allow explicit colour selection for memory map debug messages

[memmap] Allow explicit colour selection for memory map debug messages

Provide DBGC_MEMMAP() as a replacement for memmap_dump(), allowing the
colour used to match other messages within the same message group.

Retain a dedicated colour for output from memmap_dump_all(), on the
basis that it is generally most useful to visually compare full memory
dumps against previous full memory dumps.

Signed-off-by: Michael Brown <mcb30@ipxe.org>