mdadm: fix do_package failed when changed local.conf but not cleaned

reproduce steps:
1. add DISTRO_FEATURE_append = 'usrmerge' in local.conf
2. bitbake mdadm --success
3. remove DISTRO_FEATURE_append = 'usrmerge' from local.conf
4. bitbake mdadm  -- failed when do_package

it is not proper to change source Makefile during do_install by sed,
fix by pass correct config to EXTRA_OEMAKE

[YOCTO #13493]

Signed-off-by: Changqing Li <changqing.li@windriver.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>