package/swupdate: install headers at the standard location

In commit eb2f2886b2 ("package/swupdate:
add staging install"), the swupdate package was changed to also be
installed in the staging directory, as it provides an API to external
programs, through libraries and headers.

Back then the commit log said:

    This patch installs headers and libs to STAGING_DIR, to prevent a
    header name collision a subdirectory /usr/include/swupdate creates.

And indeed a custom INCLUDEDIR was specified. However, installing
swupdate headers in /usr/include/swupdate/ doesn't work well, as
progress_ipc.h (installed by swupdate) does a #include
<swupdate_status.h>, which doesn't work as swupdate_status.h is also
in /usr/include/swupdate/ instead of /usr/include/.

To address this Daniel Lang submitted a patch [0], which was proposed
to upstream swupdate to fix this header inclusion. The patch was
rejected by upstream saying that the swupdate header files should be
installed in /usr/include.

Currently, swupdate installs only 3 headers files:

  /usr/include/network_ipc.h
  /usr/include/swupdate_status.h
  /usr/include/progress_ipc.h

To the best of our knowledge, this doesn't conflict with any other
header files installed by other packages. A good proof of that is that
even Debian (which has many more packages than Buildroot) also
installs swupdate headers directly in /usr/include.

In addition, the original submitter of
eb2f2886b2 said "I'm not remember what
was a problem. Probably network_ipc.h but Im not sure now. I think
it's safe to install this headers to /usr/include w/o subdir if this
collision was met only in my environment. PS: currently I don't see
any collisions with this files."

So, we switch to installing swupdate into their standard location.

[0] https://patchwork.ozlabs.org/project/buildroot/patch/VI1P190MB0493D233ECC579510F8C0FC69FB29@VI1P190MB0493.EURP190.PROD.OUTLOOK.COM/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Daniel Lang <dalang@gmx.at>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/swupdate/swupdate.mk

@@ -230,7 +230,7 @@ endef
 define SWUPDATE_INSTALL_STAGING_CMDS
 	$(TARGET_MAKE_ENV) $(SWUPDATE_MAKE_ENV) $(MAKE) -C $(@D) \
 		$(SWUPDATE_MAKE_OPTS) DESTDIR=$(STAGING_DIR) \
-		INCLUDEDIR=/usr/include/swupdate install
+		install
 endef
 
 define SWUPDATE_INSTALL_TARGET_CMDS