at91: Switch to SD_BOOT / CONFIG_NAND_BOOT

The values CONFIG_SYS_USE_NANDFLASH and CONFIG_SYS_USE_MMC serve the
same purpose as CONFIG_SD_BOOT / CONFIG_NAND_BOOT so migrate to using
these switches instead as they're already in Kconfig.

Cc: Stelian Pop <stelian@popies.net>
Cc: Heiko Schocher <hs@denx.de>
Cc: Daniel Gorsulowski <daniel.gorsulowski@esd.eu>
Cc: Eugen Hristev <eugen.hristev@microchip.com>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/configs/at91sam9g20ek_2mmc_defconfig b/configs/at91sam9g20ek_2mmc_defconfig
index bb1908e..c341b36 100644
--- a/configs/at91sam9g20ek_2mmc_defconfig
+++ b/configs/at91sam9g20ek_2mmc_defconfig
@@ -19,7 +19,7 @@
 CONFIG_DEBUG_UART_CLOCK=132000000
 CONFIG_DEBUG_UART=y
 CONFIG_SYS_LOAD_ADDR=0x22000000
-CONFIG_SYS_EXTRA_OPTIONS="SYS_USE_MMC"
+CONFIG_SD_BOOT=y
 CONFIG_BOOTDELAY=3
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS0,115200 earlyprintk mtdparts=atmel_nand:256k(bootstrap)ro,512k(uboot)ro,256k(env),256k(env_redundant),256k(spare),512k(dtb),6M(kernel)ro,-(rootfs) root=/dev/mmcblk0p2 rw rootfstype=ext4 rootwait"