uncompress: implement uncompress_fd_to_buf ...

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

uncompress: fix uncompress_buf usage ...

uncompress_buf is used each time uncompress is called, so
make sure it is initialized correctly so that we do not
call free() on an already free pointer.

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

Add generic uncompress function ...

uncompress() has the same prototype as the various kernel decompress
functions. It automatically detects the compression type and selects
the correct uncompress function.

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