Rename --target-efi to --[enable|disable]-efi

Rename --target-efi to --enable-efi to make it similar to autoconf's
configure script options. Add the --disable-efi option for symmetry and to
make it easy for the option to change from disabled by default to enabled
by default in the future.

Suggested-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>