toolchain: remove useless indirect variables

The BR2_CONFIGURE_DEVEL_SYSROOT, BR2_CONFIGURE_STAGING_SYSROOT and
BR2_CONFIGURE_BUILD_TOOLS were used only in a few places, and it is in
fact clearer to just use their value in the various places they are
used.

The ultimate goal is to get rid of the toolchain/Makefile.in file.

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