phase1: rework MakeEnv()

phase1: rework MakeEnv()

Always set CCC and CCXX since ./staging_dir/host/bin/{gcc,g++} might be
symlinked to the ccache wrapper which uses $CCC / $CCXX to refer to the
actual compiler, even when we intend to invoke the build without ccache.

While being at it, shuffle the code around somewhat to reduce some
redundancy and to make the function a bit shorter.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>