MAKEALL: Do a sanity check on user-supplied arguments

Add a check to make sure that the user's arguments actually find a board
in boards.cfg.  Previously, if a user misspelled an argument the
argument would be discarded without warning.  For example, running
'MAKEALL -c 85xx' with the intention of compiling all Freescale 85xx
boards would instead silently discard the '-c 85xx' argument since the
proper cpu name is 'mpc85xx' and then proceed to compile all PowerPC
boards (MAKEALL's default).

Also fix an unrelated typo.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>