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/zynq_zc770_xm012_defconfig b/configs/zynq_zc770_xm012_defconfig
index 4d93206..71b379a 100644
--- a/configs/zynq_zc770_xm012_defconfig
+++ b/configs/zynq_zc770_xm012_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_OS_BOOT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_SYS_PROMPT="Zynq> "
+CONFIG_CMD_IMLS=y
 CONFIG_CMD_FPGA_LOADBP=y
 CONFIG_CMD_FPGA_LOADFS=y
 CONFIG_CMD_FPGA_LOADMK=y