agl_raspberrypi*.inc: Set KERNEL_IMAGETYPE to Image

agl_raspberrypi*.inc: Set KERNEL_IMAGETYPE to Image

Set KERNEL_IMAGETYPE to Image to ensure correct
generation of kernel7.img and successful booting
of AGL on Raspberry Pi 3 and 2 with output from
the serial console.

v2 (jsmoeller): Also adapt
   templates/machine/raspberrypi3/test/hwtest.short.environment

v3 (jsmoeller): disable CI hwtests as rpi firmware is used

Bug-AGL: SPEC-649
Bug-AGL: SPEC-650

Change-Id: If4335ffed7047fd2fbbf4bbd2dff54322cd0bf07
Signed-off-by: Leon Anavi <leon.anavi@konsulko.com>
Signed-off-by: Jan-Simon Möller <jsmoeller@linuxfoundation.org>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/9727
Tested-by: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
ci-image-build: Jenkins Job builder account <agl-jobbuilder@automotivelinux.org>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>