image: Fix Android boot image support

This patch makes the following changes:
- Set kernel entry point correctly
- Append bootargs from image to global bootargs instead
        of replacing them
- Return end address instead of size from android_image_get_end()
- Give correct parameter to genimg_get_format() in boot_get_ramdisk()
- Move ramdisk message printing from android_image_get_kernel() to
	android_image_get_ramdisk()

Signed-off-by: Ahmad Draidi <ar2000jp@gmail.com>
Cc: Tom Rini <trini@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>