Makefile: consolidate hardcoded lists of files to clean

Currently, we have two places where we list files that should always be
cleaned if they exist. One, the CLEAN_FILES variable, is from the Linux
build system and is the proper place to list files.

The other, a set of extra arguments passed in the xargs template used to
remove files with certain extensions, was introduced by 8f06f0ce
("Makefile: clean image.map") and is clearly wrong: by extending the
xargs template, we attempt to remove the files once for each batch of
arguments that xargs produces and we reduce the number of files from
stdin that xargs can include in each of its rm commands.

To fix this, put all hardcoded files into CLEAN_FILES.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>