test: cmd: fdt: Test fdt header

test: cmd: fdt: Test fdt header

Add 'fdt header' test which works as follows:
- Create basic FDT, map it to sysmem
- Print the FDT header
- Get all members of the FDT header into variable and
  verify the variables contain correct data

The test case can be triggered using:
"
./u-boot -Dc 'ut fdt'
"
To dump the full output from commands used during test, add '-v' flag.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Merged in test: cmd: fdt: Drop unused fdt_test_header_get() fdt parameter:
Signed-off-by: Simon Glass <sjg@chromium.org>