kernel-devicetree: Introduce KERNEL_DTC_FLAGS to pass dtc flags

Currently DTC_FLAGS kernel makefile parameter can be specified directly on the
command line by adding it to KERNEL_EXTRA_ARGS. However, this prevents
scripts/Makefile.lib logic from appending flags that silence dtc warnings (all
assignments done from within a makefile, to a variable specified on the command
line, are ignored).

Because of this, the do_compile log is cluttered with dtc warnings that should
only be printed when compiling with W="123":
...
/soc: node has a reg or ranges property, but no unit name
/soc/gpu: missing or empty reg/ranges property
/soc/firmware/gpio: missing or empty reg/ranges property
...

To fix this, introduce the dedicated KERNEL_DTC_FLAGS variable to hold
dtc flags and export DTC_FLAGS in the environment before generating the dtbs
(make allows "+=" operations on variables that come from the environment, so
the warnings are silenced properly).

Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
(cherry picked from commit 063b5de8)
Signed-off-by: Steve Sakoman <steve@sakoman.com>