env_mmc: Properly prefix mmc errors with '!'

The set_default_env() function from env_common.c expects either
a fully formatted error msg, e.g.: "## Resetting to default environment\n"
or an error msg prefixed with an !, in which case it will format it.

Fix the init_mmc_for_env() error messages to be prefixed with a !
this changes the bootup-log on sunxi when no mmc card is found from:

MMC:   SUNXI SD/MMC: 0
No MMC card foundIn:    serial
Out:   serial

To:

MMC:   SUNXI SD/MMC: 0
*** Warning - No MMC card found, using default environment

In:    serial
Out:   serial

Which clearly is how things should look.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>