Build the dtc binary as a static executable

This enables the dtc binary to be used without dependencies
on shared libraries, for instance, when the binary is copied
to be used in the Android kernel build process.

Test: mmm -j8 external/dtc , ldd out/host/linux-x86/bin/dtc

Change-Id: Ic566104dc8064bdea12a9d3e0c442b518dc5d2ec