Add pkg-map for redhat
Add pkg-map for redhat

The bootloader element lacks the mapping for grub-pc and extlinux
This adds it.

Change-Id: Ic7b82903f02faaab143d2bd682876bf4853fd90d
Closes-Bug: 1534387

switch-ch/diskimage-builder,takahashinobuyuki/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,openstack/diskimage-builder,zioproto/diskimage-builder,CSC-IT-Center-for-Science/diskimage-builder,takahashinobuyuki/diskimage-builder,switch-ch/diskimage-builder