adt_installer: do not install rootfs if target is not selected

adt_installer: do not install rootfs if target is not selected

Currently, if YOCTOADT_TARGETS does not contain an architecture but the
rootfs/machine settings are uncommented, then the rootfs is installed
and adt will throw an error because is not able to find the toolchain
environment script.

This patch will:
 * not allow to install a target rootfs if the toolchain for the
   target architecture is not selected;
 * uncomment the target rootfs/machine settings for the other
   architectures since it's easier for the user to just add a new
   architecture in YOCTOADT_TARGETS and have the target rootfs
   installed;

[YOCTO #5727]

(From OE-Core rev: 22351d27de76b39cb71904396e73497e36d9e1d6)

Signed-off-by: Laurentiu Palcu <laurentiu.palcu@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>