apm821xx: use lzma compression for the initramfs images

apm821xx: use lzma compression for the initramfs images

The MR24's u-boot takes it sweet time decompressing the
LZMA-packed initramfs image. A user reported that
compared to the old gzip method in v2: it "takes a ton
longer to decompress like 4\x the old boot time for
decompression".

This patch also fixes a issue with the WNDR4700's initramfs
image getting to big and causing the following u-boot crash
during the decompression:

"Uncompressing Multi-File Image ... Error: inflate() returned -5
out-of-mem or overwrite error - must RESET board to recover"

This patch fixes both issues by reverting the MR24's initramfs
compression method back to gzip. And choosing to compress the
initramfs within the initramfs image as LZMA by default.

Cc: chrisrblake93@gmail.com
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>