rpi-userland: cleanup *_CONF_OPT

This change reverts the commit 5e3b1f31
because rpi-userland is correctly built whatever the value of the
BUILD_SHARED_LIBS CMake flags.

The cset 5e3b1f31 originally intends to
fix linkage/runtime failure in xbmc package because of missing flags in
xbmc LIBS env. var. This has been fixed in the previous patch.

So, for rpi-userland package, BUILD_SHARED_LIBS does no longer need to
be forced to OFF, and can safely be driven by the CMake infrastructure.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: "Maxime Hadjinlian" <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>