cmd: Toggle the default value of CONFIG_CMD_IMLS

Having this as a 'default y' is rather annoying because it doesn't
actually compile unless other options are defined in the board header:

../cmd/bootm.c: In function 'do_imls_nor':
../cmd/bootm.c:330:7: error: 'CONFIG_SYS_MAX_FLASH_BANKS' undeclared (first use in this function); did you mean 'CONFIG_SYS_MAX_FLASH_SECT'?
   i < CONFIG_SYS_MAX_FLASH_BANKS; ++i, ++info) {

Make it 'default n' so people who develop new boards that start from a
blank defconfig have one less compilation failure to debug.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
diff --git a/configs/sama5d3xek_mmc_defconfig b/configs/sama5d3xek_mmc_defconfig
index 8b4c37a..cf57936 100644
--- a/configs/sama5d3xek_mmc_defconfig
+++ b/configs/sama5d3xek_mmc_defconfig
@@ -25,6 +25,7 @@
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMI is not set
+CONFIG_CMD_IMLS=y
 # CONFIG_CMD_FPGA is not set
 # CONFIG_CMD_LOADS is not set
 CONFIG_CMD_MMC=y