Makefile: don't use the full path to the toolchain
When rootfs_bak is passed, save the backup if not already present

The bootloader does not check for the existence of the backup, so
it is possible to have the case where "rootfs_bak" is present on
the kernel command line but no backup is available.
Only process one loop= parameter
If kernel_bak is passed, mount the backup kernel when possible
Load the backup rootfs named rootfs.bin.bak instead of rootfs.bin.old

This changes makes it consistent with the name of the backup of
the kernel: vmlinuz.bin.bak.
When doing a rootfs upgrade, backup also the SHA1 file
Simplify prototypes of __mount and __multi_mount