dtc: compile fdtget ...

We need fdtget in subsequent patches, so compile it.
Unfortunately this is not straight forward. fdtget needs libfdt,
but the Kernel Build System is not prepared for compiling
libraries for the host or binaries from files in multiple directories.
This patch moves the libfdt files to the dtc toplevel directory and
compiles fdtget as a binary from multiple source files but from
the same directory.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>