cmd, fdt: add subcommand "get" to fdt header

cmd, fdt: add subcommand "get" to fdt header

store fdt header member with name <member> in U-Boot
Environment variable with name <var>.

for example to get the total length of the fdt and store
it in filesize, call:

fdt header get filesize totalsize

For membernames look into fdt header definition at
scripts/dtc/libfdt/libfdt.h

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>