./build: enable to mask some tasks.

If we list all the packages that have a release task we have:
    $ ./build release list
    Available packages for task 'release':

    roms
    src
    u-boot-libre
    website

If for some reasons we don't want to have u-boot-libre shown (because
for instance it doesn't build and you want to avoid building it as
part of the './build release all' command, then you can mask it by
removing the executable permission:
    $ chmod -x resources/packages/u-boot-libre/release

And it then doesn't show up anymore:
    $ ./build release list
    Available packages for task 'release':

    roms
    src
    website

Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@cyberdimension.org>
Acked-by: Adrien 'neox' Bourmault <neox@gnu.org>

build

@@ -56,6 +56,7 @@ list_tasks_paths() {
 	find resources/packages \
 		-mindepth 2 -maxdepth 2 \
 		-type f \
+		-executable \
 		-name "${task}" \
 		-printf "%P\n"
 }