package/pkg-generic.mk: exclude the staging sub-directory

Now that we are checking the host directory changes throughout all
installation steps and not just during the "host installation step",
it means that changes done within the staging directory (which is a
subdir of the host directory) are also visible in the
.files-list-host.txt file.

Note that this problem already potentially occurs if a host package is
installing files in the staging directory: they would be listed in
.files-list-host.txt even without the changes in this series.

To fix this up, we simply exclude files that are beneath the
$(STAGING_SUBDIR). Note that we do that in all cases, so when
searching $(HOST_DIR), $(HOST_DIR)/$(STAGING_SUBDIR) is excluded, but
when searching $(TARGET_DIR), $(TARGET_DIR)/$(STAGING_SUBDIR) is
excluded, and when search $(STAGING_DIR),
$(STAGING_DIR)/$(STAGING_SUBDIR) is excluded. This is not a problem in
practice since $(TARGET_DIR)/$(STAGING_SUBDIR) and
$(STAGING_DIR)/$(STAGING_SUBDIR) don't exist, but it's not very
nice. However, it allows to keep the code simple.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>