pakage/rpi-userland: add and install our bcm_host.pc

This allows pkg-config-aware packages to include headers from, and
link against Broadcom VideoCore host API library.

bcm_host.pc copied almost as-is from:
    http://cgit.collabora.com/git/user/pq/android-pc-files.git/tree/pkgconfig?h=raspberrypi


with only a fix to the prefix (we're using /usr).

[Peter: s/TARGET/STAGING/]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>