ARM: at91/defconfig: refresh at91sam9260_9g20_defconfig

Select CONFIG_EMBEDDED as it it useful for that board.

Also select CONFIG_MTD_UBI to really enable UBIfs (CONFIG_FS_UBIFS depends on
it).

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>