config_cmd_defaults.h: new header for common u-boot command defaults

We have config_defaults.h which are random configuration settings that
everyone gets by default.  We also have config_cmd_default.h which is a
recommended list of defaults but boards have to opt into.  Now we have
config_cmd_defaults.h which is a list of defaults that everyone gets
and has to actively opt out of.

For now, we populate it with the bootm command which previously was
unable to be disabled.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>