When upgrading from Buildroot 2021.2 to 2021.08, the following error
occurs:

 $ make buildroot
 ...
   GEN     /home/jerome/work/optee_repo_qemu_v8/out-br/Makefile

 Your PATH contains spaces, TABs, and/or newline (\n) characters.
 This doesn't work. Fix you PATH.
 make[1]: *** [Makefile:23: _all] Error 2
 make[1]: Leaving directory '/home/jerome/work/optee_repo_qemu_v8/out-br'
 make: *** [common.mk:323: buildroot] Error 2

The space comes from the line that appends the toolchain to the PATH in
optee_rust_examples_ext.mk. Fix it.

Signed-off-by: Jerome Forissier <jerome@forissier.org>