Revert "FROMLIST: kbuild: Add dtc flag test"

This reverts commit 47b5470f.

The variable DTC_FLAGS is a recursively expanded makefile variable which
means that it is re-expanded every time it is referenced. In other
words, a try-run is performed every time this variable is referenced
which is wasteful because the result will always be the same.  In
addition, if dtc is invoked with $(call if_changed_dep,dtc,dtb),
DTC_FLAGS is apparently expanded three times. As a result, we observe 3
try-runs per dtc-option of which there are 8. This means that every
compilation of a device tree file is preceded by 24 try-runs! Every
invocation of dtc turns into 25 invocations!  Additionally, it appears
as if Make prohibits parallelization if a recipe contains a reference to
the $(shell ....) function.

Bug: 198176779
Change-Id: Iae1a1d2266563727f1a7b67660c7b8fa7e1c7247
Signed-off-by: Daniel Mentz <danielmentz@google.com>
(cherry picked from commit e4f26478)