decompressors: Use malloc/free wrappers ...

The decompressors are used both in a regular image and also for image
decompression. Both need different malloc implementations. Using
malloc/free directly in the decompressor code easily leads to include
file conflicts, so use MALLOC/FREE which can be defined correctly for
the two different usecases.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Add xz decompression support ...

This adds xz decompression support from the kernel. Both compressing
the barebox binary with xz and decompressing xz files on the commandline
is supported.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>