Linux26 requires "mkimage" from u-boot to be in the PATH

Linux26 requires "mkimage" from u-boot to be in the PATH
if an uImage is to be built for arm, avr32, blackfin and sh.

PATH has therefore been set to have $(STAGING_DIR)/usr/bin
as the first item allowing linux to find mkimage.

This patch will, if an uImage is built, create a "u-boot-toóls"
directory in the PROJECT_BUILD_DIR directory.
$(STAGING_DIR)/usr/bin/mkimage is copied to this directory
and the PATH variable will now include "u-boot-tools"

If an uImage is built, then the linux build will now depend on "u-boot".