OptionalObsoleteFiles: Don't mark /usr/lib/debug/boot directory obsolete

The intent of the currect code is to ignore anything under
/usr/lib/debug/boot/*.  But we also should make sure that
/usr/lib/debug/boot directory is also ignored and is not marked
obsolete.  If we don't do that, `make DBATCH_DELETE_OLD_FILES
delete-old` will try to rmdir(1) this directory, which will cause an
error, since /usr/lib/debug/boot may have nested directories like
kernel/ and modules/.

Reviewed by:	markj
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D55077

tools/build/mk/OptionalObsoleteFiles.inc

@@ -1443,8 +1443,8 @@ OLD_DIRS+=usr/include/fs/cuse
 
 .if ${MK_DEBUG_FILES} == no
 .if exists(${DESTDIR}/usr/lib/debug)
-DEBUG_DIRS!=find ${DESTDIR}/usr/lib/debug -mindepth 1 \
-    -type d \! -path "${DESTDIR}/usr/lib/debug/boot/*" \
+DEBUG_DIRS!=find -E ${DESTDIR}/usr/lib/debug -mindepth 1 \
+    -type d \! -regex "${DESTDIR}/usr/lib/debug/boot(/.*)?" \
     | sed -e 's,^${DESTDIR}/,,'; echo
 DEBUG_FILES!=find ${DESTDIR}/usr/lib/debug \
     \! -type d \! -path "${DESTDIR}/usr/lib/debug/boot/*" \! -name "lib*.so*" \