configs: fs.py can take multiple disk images on most ISAs

configs: fs.py can take multiple disk images on most ISAs

All ISAs except SPARC can now take multiple disk images by passing
the --disk-image option multiple times.

Before this patch, several ISAs automatically mounted a secondary disk
called "linux-bigswap2.img", which had to be in M5_PATH even if the end
user did not want more than one disk. This was the case for for example
for X86 but not ARM.

This change was done to:

* allow ARM to have a second disk image in fs.py, which was not possible,
  and allow other ISAs like X86 and ARM to take any number of disk images

* provide a simpler, more intuitive CLI interface that does not require
  magic disk images to be present in M5_PATH to work for ISAs such as X86.

  Linux does not need that secondary image to boot correctly, so it is
  more friendly to support a minimal setup that requires the least amount
  of binaries to boot, and let supply the second image manually only if
  they need it.

* make fs.py --disk-image work more similarly across all ISAs

SPARC was left with a single disk only because its setup was a bit more
complex and would require further testing.

Change-Id: I8b6e08ae6daf0a5b6cd1d57d285a9677f01eb7ad
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23671
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>