ANDROID: build.config: Set DTC to absolute path

The device tree compiler specified by $(DTC) is actually used as a
prerequisite for the following .dtb makefile rule:

$(obj)/%.dtb: $(src)/%.dts $(DTC) FORCE

Ensure that it's set to an absolute path so that make can actually find
it. Otherwise, make will not instantiate that rule, because it can't
find dtc.

Bug: 198176779
Fixes: 023411cb ("ANDROID: Use DTC from the hermetic toolchain")
Signed-off-by: Daniel Mentz <danielmentz@google.com>
Change-Id: I4e190192b39c4cf18c6412cc85895ed1ce2ec6dd