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:
  crossystem ''
  crossystem '=cros_debug'
  crossystem '?cros_debug'

The following command line should warn that you can't use both = and ?:
  crossystem cros_debug?=0
(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)

Review URL:

Change-Id: I2e2851515f4914b16aba64065600fb92d9ad1a63
(cherry picked from commit f27583f08361522909e49155d8b14671f644ec85)

Review URL:
1 file changed