tree: respect the package's own default CFLAGS

The tree Makefile defines default CFLAGS form Linux (-DLINUX, etc)
which are lost if we don't manually include them in the final CFLAGS.

Also break the dependency on EXTRA_OEMAKE containing '-e', disable the
unused configure task and, since package has a very short description,
set SUMMARY instead of DESCRIPTION.

Signed-off-by: Andre McCurdy <armccurdy@gmail.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>