kobo/uimage: add git submodule

git://ventus.caztech.com/XCSoar-Kobo-Build
for compiling uImage to support stock and
USB-host behavior
kobo.mk: add KOBO_UIMAGE=y option to include uImages in KoboRoot.tgz

This allows for the correct uImage to be installed by the
KoboMenu so it can support PC Connect or USB Host mode
README: update for Kobo build options information