libfdt: Introduce fdt_create_with_flags()

There is a need to be able to specify some options when building an FDT
with the SW interface. This can be accomplished with minimal changes by
storing intermediate data in the fdt header itself, in fields that are
not otherwise needed during the creation process and can be set by
fdt_finish().

The fdt.magic field is already used exactly this way, as a state to
check with callers that the FDT has been created but not yet finished.

fdt.version and fdt.last_comp_version are used to make room for more
intermediate state. These are adjacent and unused during the building
process. last_comp_version is not yet used for intermediate state, but
it is zeroed and treated as used, so as to allow future growth easily.

A new interface, fdt_create_with_flags() is added, which takes 32-bit
flag value to control creation.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Message-Id: <20190509094122.834-3-npiggin@gmail.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>