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/xtfpga_defconfig b/configs/xtfpga_defconfig
index f479c1a..4f77e23 100644
--- a/configs/xtfpga_defconfig
+++ b/configs/xtfpga_defconfig
@@ -7,6 +7,7 @@
 CONFIG_AUTOBOOT_KEYED=y
 CONFIG_AUTOBOOT_PROMPT="Autobooting in %d seconds, press <SPACE> to stop\n"
 CONFIG_AUTOBOOT_STOP_STR=" "
+CONFIG_CMD_IMLS=y
 CONFIG_CMD_ASKENV=y
 CONFIG_CRC32_VERIFY=y
 CONFIG_CMD_SAVES=y