configs: make --disk-images optional in fs.py

configs: make --disk-images optional in fs.py

The main applications are to run baremetal programs and initramfs Linux
kernel.

Before this patch, disks() calls in makeArmSystem would throw:

IOError: Can't find file 'linux-aarch32-ael.img' on M5_PATH.

In order to achieve this, this commit also removes the default hardcoded
disk image basenames.

For example, before this commit, running without a --disk-image in X86
would automatically search for an image with basename x86root.img in
M5_PATH, which means we would either have to ignore any disk image error,
or else running without disk images would fail.

After this commit, you would have to pass --disk-image x86root.img to
achieve the old behaviour.

Change-Id: I0ae8c4b3b93d0074fd4fca0d5ed52181c50b6c04
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/27867
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>