Makefile: Build a separate .dtb for TPL

At present both SPL and TPL use the same devicetree binary. While there
is logic to run fdtgrep separately on each one, it does not actually
happen.

Add a new TPL rule and use that instead. Make this rule conditional on
there actually being a TPL. Do the same for SPL for consistency.

Note that the SPL and TPL dtbs are build by a Makefule rule used for
U-Boot proper. This is the 'dtbs' target in dts/Makefile. So the check
for CONFIG_TPL_BUILD in cmd_fdtgrep never actually works at present.

We don't support CONFIG_OF_EMBED for TPL at present.

Signed-off-by: Simon Glass <sjg@chromium.org>