rootfs_rpm.bbclass: Handle multilib configures with different OS values

rootfs_rpm.bbclass: Handle multilib configures with different OS values

[ CQID: WIND00404082 ]

It is possible for the TARGET_OS to change dependending on the multilib used
for a given package build.  mips64 has two potential TARGET_OS values: linux
and linux-gnun32.  The RPM and Smart setup needs to distinguish between the
two otherwise packages of the "non-default" TARGET_OS may be ignored as
incompatible.

(From OE-Core rev: aa760ff7dee4029c30c9de9fc0b640149e4a4c77)

Signed-off-by: Mark Hatle <mark.hatle@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>