cfb_console: align fields in gzipped .bmp files

.bmp files contain 32-bit integers aligned at offsets of +2, +6,
et cetera within the bmp_header structure (see include/bmp_layout.h).

Support for gzip-compressed .bmp files is present in the cfb_console
display subsystem by uncompressing them prior to use.

This patch forces the in-memory header to be aligned properly
for these compressed images by extracting them to a 2-byte
offset in the memory returned by malloc. Since malloc will always
return a 4-byte aligned value, this forces the .bmp header
fields to be naturally aligned on 4-byte addresses.

Refer to these files for more details:
	doc/README.displaying-bmps

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>