open2300: fix build failure

This patch fixes the open2300's build-system, which is a handwritten
Makefile, by removing the hard-coded include and library locations and
making them pointing to the systroot.

Fixes:
  http://autobuild.buildroot.org/results/072/0724ffa1355f5564b1fcd9d6e3c9ff2162d20f44/build-end.log



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>