sh: fix memory corruption of unflattened device tree

unflatten_device_tree() makes use of memblock allocation, and
therefore must be called before paging_init() migrates the memblock
allocation data to the bootmem framework. Otherwise the record of the
allocation for the expanded device tree will be lost, and will
eventually be clobbered when allocated for another use.

Signed-off-by: Rich Felker <dalias@libc.org>