Modify geninstaller for uefi booting
Modify geninstaller for uefi booting

Switch image to use overlay partition and use uefi layout and boot loader
configuration.

Signed-off-by: Ryan Harper <7c8ea25f69f5166c51151e84bf5db678c2384828@canonical.com>

CanonicalLtd/subiquity,CanonicalLtd/subiquity