cmd: spl: provide address and size of prepared FDT in environment

Writing prepared FDT to persistent storage should be possible in
scripts. Create environment variables containing address and size
of the updated FDT. Scripts can use these variables after running
'spl export fdt ...' command to write the new blob to persistent
storage.

Signed-off-by: Anatolij Gustschin <agust@denx.de>