pkg-cmake.mk: do not hardcode absolute path in toolchainfile.cmake

The patch allows sharing or moving the toolchains.

This is a step toward making the toolchain/sdk relocatable.

Closes #6818

[Peter: reword comment as suggested by Thomas]
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Uwe Strempel <u.strempel@googlemail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>