Add (Freescale) elftosb host package

Elftosb is a Freescale toolset to build binary image files (like U-Boot.sb)
to be used with CPUs imx23, imx28, stp37xx

thomas.petazzoni@free-electrons.com:
 * Add -m 0755 when installing binaries
 * the _CLEAN_CMDS is not supposed to uninstall things from the
   HOST_DIR.
 * Use 'host-generic-package' instead of 'GENTARGETS,host'

Signed-off-by: Eric Jarrige <eric.jarrige@armadeus.org>
Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>