qextserialport: fix static build

By default, qextserialport will attempt to build a shared library.
qesp_static has to be set in CONFIG to build static. Note that
static+shared is not supported, in that case we just build shared.

The install target commands also have to be gated in that case,
because the *.so files don't exist. For completeness we both set
QEXTSERIALPORT_INSTALL_STAGING to NO and don't define
QEXTSERIALPORT_INSTALL_TARGET_CMDS for static builds, although one
of them would be sufficient.

Fixes:
http://autobuild.buildroot.net/results/c9233ad71fd60d0e6a85731a8bd4e598bd84947a



Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>