If CONFIG_CMD_IMD is not set there is no imd_command_setenv in the
barebox binary that can be linked to. Although the whole imd infrastructure
will be removed by the linker later in the build process as soon as it
figures out that nothing inside barebox is using it, we still have to
provide a dummy function to keep the build going.

Fixes:
In function `imd_command': undefined reference to `imd_command_setenv'

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>