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/ls2080ardb_defconfig b/configs/ls2080ardb_defconfig
index c44ac48..0fd3fee 100644
--- a/configs/ls2080ardb_defconfig
+++ b/configs/ls2080ardb_defconfig
@@ -9,6 +9,7 @@
 CONFIG_BOOTDELAY=10
 CONFIG_USE_BOOTARGS=y
 CONFIG_BOOTARGS="console=ttyS1,115200 root=/dev/ram0 earlycon=uart8250,mmio,0x21c0600 ramdisk_size=0x2000000 default_hugepagesz=2m hugepagesz=2m hugepages=256"
+CONFIG_CMD_IMLS=y
 CONFIG_CMD_GREPENV=y
 CONFIG_CMD_EEPROM=y
 CONFIG_CMD_GPT=y