meta-mender-e31x: conf: Reduce total SD image size to 7.2G
meta-mender-e31x: conf: Reduce total SD image size to 7.2G

The shipping SD card with E310 is 8G with 7.2G usable space,
So changing the size from 7.4G to 7.2G

Signed-off-by: Sugandha Gupta <d84b5530ef1fc2430e5b91666d65ccc4e6d6f16e@ettus.com>

EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus,EttusResearch/meta-ettus