xtensa: clean up bootable image build targets

Currently xtensa uses 'zImage' as a synonym of 'all', but in fact xtensa
supports three targets: 'Image' (ELF image with reset vector), 'zImage'
(compressed redboot image) and 'uImage' (U-Boot image).
Provide separate 'Image', 'zImage' and 'uImage' make targets that only
build corresponding image type. Make 'all' build all images appropriate
for a platform.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>