rootfs_rpm.bbclass: normalize arch output of list_installed_packages

rootfs_rpm.bbclass: normalize arch output of list_installed_packages

list_installed_packages should output uniform formatted archs for all
packaging system(deb, rpm and ipk), for they are used by common code to
grab the package infos. Otherwise, it will lead some -dbg packages to be
missed processing in come cases.

It's introduced by commit: fc985f51, package_rpm.bbclass: fix the arch
(replace "-" with "_").

Fixed by replacing "_" back to "-" in list_installed_packages, make it
output real arch values which can be safely used.

(From OE-Core rev: 554f2436435bf96c6735827f491c1b78b198c016)

Signed-off-by: Ming Liu <ming.liu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>