layerscape: add a make menuconfig known issue in README

This patch is to add a known issue of make menuconfig
in README file. This is also a common issue for OpenWrt.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>

target/linux/layerscape/README

@@ -61,6 +61,12 @@ $ ./scripts/feeds install -a
   | [*] ls2088ardb-armv8_64b  --->                                  |
   +-----------------------------------------------------------------+
 
+Note: The first time make menuconfig would create a .config file which
+would include all dependencies for selected target. After that, make
+menuconfig still could be used to modify packages. If want to change
+other target, please remove .config and make menuconfig to select again.
+Otherwise the packages selected in .config would be a mess.
+
 2.2 make (or make -j<n>)
 ------------------------