aspeed-g6: defconfig: Add support for USB flash drives

Add support to detect USB flash drives and create the /dev/sd* devices.
Also add support for vfat to support USB drives formatted as FAT32.
This support will be used to enable firmware updates via USB flash
drives where the firmware image is stored in the USB drive and it's
plugged into the BMC USB port for AST2600 systems.

Change-Id: I82151eb409e48324607efd65f79fdbe4df36ced3
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>