bootstd: Handle a few special cases in cmdline_set_arg()

bootstd: Handle a few special cases in cmdline_set_arg()

Two bugs have appeared:

- arguments can have an equals sign embedded in them, which must be
  considered part of the value
- arguments must fully match the name; partial matches should be
  ignored

Fix these and add a test to cover both.

Signed-off-by: Simon Glass <sjg@chromium.org>