package/pkg-qmake: new qmake package infrastructure

package/pkg-qmake: new qmake package infrastructure

This provides generic functions for Qt5 qmake based packages. It will
make it possible to remove lots of redefinition of
QT5_xxx_{CONFIGURE|BUILD|INSTALL_STAGING}_CMDS. Additionally it
provides a generic target install method which will make most of the
package specific commands obsolete.

This is done by re-running the install step of the qmake generated
Makefile with the package build directory prepended (to the
staging/host path). Even though this does create lengthy pathes it
allows for easy separation of the staging files from the host destined
files by just omitting the resulting BUILD_DIR+HOST_DIR path from the
following rsync call to the real target folder.  The cleanup of many
files we dont want in target is deferred to the target-finalize
step. In addition to what's being removed already, we also have to
cleanup some Qt5 specific files (prl) and the documentation directory.

This approach was chosen over copying all files recorded in the pkg-files-list
after some discussion which Thomas Petazzoni summed up:
"We don't yet use pkg-files-list really as part of the build
process anywhere, I feel a bit more comfortable at this point with what
Andreas is proposing."

Thanks to this infrastructure, it will be possible to get rid of the
many conditional install commands because qmake already takes care of
this when generating the Makefile install targets with the given or
autodetected configure options of each package.

However, custom install steps may have to remain in cases where a
particular Buildroot option has no corresponding setting in the
packages configuration options.

Signed-off-by: Andreas Naumann <anaumann@ultratronik.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>