Makefile: move determination test for OS to Makefile.d

Move the test code for OS detection in a extra directory to split it
from the main flashrom code.

flashrom-stable:
* Use `tail -n` to be standard compliant
* Fix some typos in comment

Change-Id: Id911f17f4100f242e1fde10d23a8459ddf38b369
Signed-off-by: Thomas Heijligen <thomas.heijligen@secunet.de>
Original-Reviewed-on: https://review.coreboot.org/c/flashrom/+/58015
Original-Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/72241
Reviewed-by: Nico Huber <nico.h@gmx.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
diff --git a/.gitignore b/.gitignore
index bcb11b7..b824e15 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,5 @@
 *.d
+!Makefile.d/
 *.o
 /.features
 /.dependencies