Makefile: don't hang the build if there are no file lists

Makefile: don't hang the build if there are no file lists

In very limited configurations, it is possible to have a case where no
.files-list-staging.txt files are created. In this case:

    cat $(sort $(wildcard $(BUILD_DIR)/*/.files-list-staging.txt)) > \
        $(BUILD_DIR)/packages-file-list-staging.txt

becomes:

    cat > \
        $(BUILD_DIR)/packages-file-list-staging.txt

which of course makes the build hang.. forever.

So we fix this by checking the list is not empty. To keep the code
readable, we introduce an intermediate variable to store the list of
these files.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
[yann.morin.1998@free.fr: always create the file, even if empty]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>