kernel-2.eclass: Migrate CROSSCOMPILE_OPTS=headers-only -> USE=headers-only

kernel-2.eclass: Migrate CROSSCOMPILE_OPTS=headers-only -> USE=headers-only

CROSSCOMPILE_OPTS is a USE_EXPAND of a single item: headers-only.
Convert it to a global USE flag instead.

Mechanical ebuild rename done as:
    $ sed -e 's@crosscompile_opts_headers-only@headers-only@g' \
        -i $(git grep -l headers-only)

'headers-only' flag is used by crossdev to bootstrap stage1 compiler
before libc is available.

crossdev switched to USE=headers-only in =sys-devel/crossdev-20171230.

Bug: https://bugs.gentoo.org/642712
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>