package/Makefile.in: fix incorrect comment

The TARGET_{CC,CXX,LD,...} variables no longer contain any --sysroot
option, since we're now using a toolchain wrapper for external
toolchains.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>