greybus: Support building greybus on host PC

Introduce INSTALL_MOD_PATH to allow for building and installing of the
greybus modules from a different location.

This lets you build the greybus modules on a PC and then install the
modules to an SD card in the appropriate location relative to the SD such
as /media/sdcard/lib/modules/version and subsequent running of depmod in
the same location. If INSTALL_MOD_PATH isn't specified the default
behaviour of installing and depmoding to /lib/modules/version is
maintained.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>