configs: drop "mmc" in engicam_imx6ul_isiot_mmc_defconfig name

_mmc on defconfig files in engicam need to drop and same follow on
other engicam defconfig files.

So, this patch rename engicam_imx6ul_isiot_mmc_defconfig to
engicam_imx6ul_isiot_defconfig which is missing in commit "board: Add
support for Engicam Is.IoT MX6UL SOM" (sha1:
0235bc61766bd0d4fecfbd686cd807992ba5f37d)

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>