GNU Make 4.4 will pass TARGET to the sub-make of UNDEF_REFS where it can
be later used as link TARGET by Genode's build system. Before 4.4 TARGET
was undefined in the sub-make leading to the correct result.
This patch enables the tool/dde_linux/create_dummies tool to operate on
a out-of-tree build directory. The Linux source tree is determined by
examining the build directory's Makefile.
Issue #4188
The new tool `create_dummies` easily lists missing references,
while porting subsystems of the Linux kernel to Genode, e.g., device drivers.
Moreover it can automatically generate dummy implementations for functions
and global variables.
Fixes#4156
