createrepo-c: fix createrepo-c failed in nativesdk

In sdk, call createrepo-c failed with:
...
$ createrepo_c --update ./test_repo/rpm
Directory walk started Critical: Failed to detect compression for file
./test_repo/rpm/cortexa72/hello-2.10-r0.cortexa72.rpm: magic_load() failed: could not find any valid magic files!
...

Since commit [ea666fbc createrepo-c: set path to magic database for
native and nativesdk] applied, the MAGIC is incorrectly assigned.

The variable datadir will be expanded automatically for nativesdk,
do not need to add prefix ${SDKPATHNATIVE} to MAGIC

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 54368f1b)
Signed-off-by: Anuj Mittal <anuj.mittal@intel.com>