Fix bootloader packages for aarch64
Fix bootloader packages for aarch64

Due to the arm naming convention, building centos images for arm64 and
aarch64 does not yield the same result. In order to locate grub2 on
aarch64 the correct mapping is added.

Change-Id: I1bb227b2523e420e394fec8c52c6c79fcdd31c53
Closes-Bug:#1789414
Signed-off-by: Charalampos Kominos <fb27d92665713ab0fcfebb9d305200781327189c@enea.com>

switch-ch/diskimage-builder,openstack/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder