Add error checking for poorly-formed crossystem args
The following command lines should cause crossystem to fail with a
warning about a poorly formed parameter:
The following command line should warn that you can't use both = and ?:
(that is, it warns, not just compares with '=0')
The following should print 'UNEQUAL'
crossystem cros_debug? || echo UNEQUAL
(because it's comparing cros_debug with an empty string)