rpm: realpath is required before expanding _dbpath in chroot

rpm: realpath is required before expanding _dbpath in chroot

A regression is introduced by commit 66573093:
[ rpm: Fix rpm relocation macro usage ]

_usr turned out to be a relative path to support dyanmic config after
that, but it's being used somewhere as a indicator to locate substrings,
so we must get the real path of it in advance.

Signed-off-by: Ming Liu <ming.liu@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>