rpm-pkg: add %post section to create initramfs and grub hooks

/sbin/installkernel is used to insure grub hooks are
inserted and the initramfs is created on the
target system.

The invokation installkernel will work with any
kernel as long as:
- /sbin/installkernel exists
- the kernel and sysem map files are readable

Signed-off-by: Mike Marciniszyn <mike.marciniszyn@intel.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>