In theory we can overwrite a builtin command with a script. However,
I don't know a single case where this has been done. Scripts are
often more unflexible than commands so it's unlikely that a script
can extend the functionality of a builtin command. Moreover, the
internal command is no longer accessible once it's overwritten by
a script.

Invert this logic so that a builtin command can overwrite an existing
script. This will help when the 'boot' script is converted to a
builting command. Then with old environments the builtin command will
be used instead of the script.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>