image.mk: keep underscores when sanitize PROFILE

image.mk: keep underscores when sanitize PROFILE

Underscores don't harm in filenames, however the sanitize function from
version.mk replaces them, causing inconsistent filenames.

A fix tried to solve this previously via
dfe99645dbdd4460f394beaa3a05091cd40b5942 however I did not look close
enough:

openwrt-ath79-generic-8dev-carambola2.manifest # current
openwrt-ath79-generic_8dev-carambola2.manifest # patched
---------------------^

Eventually the sanitization of PROFILES *could* be removed as more and
more profiles follow the device tree approach of vendor_model, neither
containing upper case letters nor spaces.

Signed-off-by: Paul Spooren <mail@aparcar.org>