rpm: write macros under libdir

If we create a macros file under etc/, the populate_sdk task would
fail if 'package-management' is in SDKIMAGE_FEATURES and nativesdk-rpm-build
is installed.. The error message is like below:

  unable to place /.../sdk/image/etc/rpm in final SDK location

This is because it's trying to move the etc/rpm dictory into the
host sysroot but the <host_sysroot>/etc/rpm has already exists.

To solve this problem, avoid creating /etc/rpm/macros for nativesdk-rpm-build,
use ${libdir}/macros instead. In this way, the macros file is hold
in nativesdk-rpm. As nativesdk-rpm-build depends on nativesdk-rpm,
the 'rpmbuild' command inside SDK can still correctly find find-debuginfo
binary.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>