Add a test for the button_cmd feature. This validates that commands can
be mapped to two buttons, that the correct command runs based on which
button is pressed, that only 1 command is run, and that no command runs
if button_cmd_0_name is wrong or unset.

Additionally, fix a potential uninitialised variable use caught by these
tests, the btn variable in get_button_cmd() is assumed to be null if
button_get_by_label() fails, but it's actually used uninitialised in
that case.

CONFIG_BUTTON is now enabled automatically and was removed when running
save_defconfig.

Fixes: e761035b ("boot: add support for button commands")
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>