capsule: Makefile: add the generated files to CLEAN_FILES list

A certain set of capsule files are now generated as part of the
sandbox build. Add these files to the CLEAN_FILES list for deletion on
invoking any of the cleanup targets.

Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canoncal.com>
Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>