powerpc/ps3: Add check for otheros image size

The ps3's otheros flash loader has a size limit of 16 MiB for the
uncompressed image.  If that limit will be reached output the
flash image file as 'otheros-too-big.bld'.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/897c2a59-378e-7c9b-3976-d0a0def90913@infradead.org