buildsys: Allow selecting drivers during cmake configuration

Although at the moment there is only the USB host driver, it still makes
sense to have an infrastructure for configuring which drivers to compile
in.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
NFM: buildsys: Do not use pkgconfig for finding libusb

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>