rootfs_rpm.bbclass: maybe no rpm postinst script

rootfs_rpm.bbclass: maybe no rpm postinst script

There maybe no rpm postinst script (e.g., core-image-minimal), then the
"*" is not expanded, and there would be error:

head: cannot open `rpm-postinsts/*' for reading: No such file or directory

Check whether it exists or not will fix the problem.

[YOCTO #3172]

(From OE-Core rev: 966c72e00c8d378d7d189f0e4b626f6782d23a25)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>