layerscape: update build guide in README

Added package feeds updating and installing in build guide
of README.

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

target/linux/layerscape/README

@@ -12,6 +12,10 @@ Layerscape Quick Start
 
 2. Build
 --------
+Before configuration and build, update and install package feeds.
+
+$ ./scripts/feeds update -a
+$ ./scripts/feeds install -a
 
 2.1 make menuconfig
 -------------------