configs: at91: Remove CONFIG_SYS_EXTRA_OPTIONS assignment

To remove the assignment of CONFIG_SYS_EXTRA_OPTIONS option,
which is deprecated, use the CONFIG_XXXX_BOOT options to
indicate the boot media, and the SoC is selected by the board.

Signed-off-by: Wenyou Yang <wenyou.yang@microchip.com>
diff --git a/configs/at91sam9n12ek_mmc_defconfig b/configs/at91sam9n12ek_mmc_defconfig
index 4c554f2..cc567c4 100644
--- a/configs/at91sam9n12ek_mmc_defconfig
+++ b/configs/at91sam9n12ek_mmc_defconfig
@@ -4,7 +4,7 @@
 CONFIG_SYS_MALLOC_F_LEN=0x2000
 CONFIG_DEFAULT_DEVICE_TREE="at91sam9n12ek"
 CONFIG_DEBUG_UART=y
-CONFIG_SYS_EXTRA_OPTIONS="AT91SAM9N12,SYS_USE_MMC"
+CONFIG_SD_BOOT=y
 CONFIG_BOOTDELAY=3
 # CONFIG_CONSOLE_MUX is not set
 CONFIG_SYS_CONSOLE_IS_IN_ENV=y