qextserialport: use 'make install' to install to staging

qmake hardcodes the path to the sysroot in the install commands, so
we can't use it for target. But it's perfectly usable for staging.

By using 'make install', we get:
- the extserialport.prf file is installed to the mkspecs directory and
  libqextserialport.prl in staging/usr/lib, so qmake can do its magic
  and add the necessary compiler options;
- it also works for static build, when *.so files don't exist.

The QExtSerialPort and qextserialport.pc files are created by
Buildroot so they still have to be installed explicitly. Note that
upstream installs in the QtExtSerialPort directory, not QExtSerialPort,
so we follow that decision.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
[Thomas: also adjust 0003-pkgconfig.patch to fix the header path.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>