fdt: send all dtc output to stderr

fdt: send all dtc output to stderr

Commit 896bbb5 "fdt: avoid bad MAKEALL status" added logic to capture
the result code from dtc by echoing $? to stdout and capturing it using
$(). However, dtc emits some diagnostics to stderr and some to stdout.
The diagnostics send to stdout ended up getting captured via $() rather
than being echo'd to the user. This caused those diagnostics to be passed
to the exit command, which would then fail with the following cryptic
error message:

    /bin/sh: line 1: exit: too many arguments

Solve this by redirecting all dtc output to stderr so that $() does not
capture it. This allows the user to see the actual error message from dtc.

Cc: Wolfgang Denk <wd@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: albert.u.boot@aribaud.net
Cc: Tom Warren <twarren@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>