env: Fix operation of 'make environ'

This was broken by the recent environment refactoring. Specifically:

$  make environ
scripts/Makefile.build:59: tools/environ/Makefile: No such file or directory
make[1]: *** No rule to make target 'tools/environ/Makefile'.  Stop.
make: *** [Makefile:1469: environ] Error 2

Fix this by updating the Makefile and adjusting the #include filesnames in
two C files.

Fixes: ec74f5f9 (Makefile: Rename 'env' target to 'environ')
Reported-by: Måns Rullgård <mans@mansr.com>
Signed-off-by: Simon Glass <sjg@chromium.org>