enable it only if a compression is enabled
support gzip, bzip2 and lzo

you will be able to choose which compression to use

-rw-r--r--  1 root root    8436 Dec 15 01:35 barebox_default_env
-rw-r--r--  1 root root    2782 Dec 15 01:35 barebox_default_env.bz2
-rw-r--r--  1 root root    2691 Dec 15 01:38 barebox_default_env.gz
-rw-r--r--  1 root root    3262 Dec 15 01:38 barebox_default_env.lzo

with using gzip and the default env we can save 5.6KiB (5,745 bytes)
with using bzip2 and the default env we can save 5.5KiB (5,654 bytes)
with using lzo and the default env we can save 5.1KiB (5,174 bytes)

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>