bootloader: Add an arguments_count field to stage2_args

Some boot platforms pass a non-NULL-terminated list of args
to the loader, so store the count here to avoid having to copy
the list itself.
bootloader: Set stage2_args:arguments_count when needed
bootloader: Use stage2_args:arguments_count

The code probably doesn't work correctly yet though,
I believe only the last line will be taken into account.
U-Boot: pass around argv and /chosen/bootargs

Not really handled correctly yet, but at least they are passed.
Style fixes