lib: fdtdec: Parse the gzip/lzo headers only when dependencies have met

lib: fdtdec: Parse the gzip/lzo headers only when dependencies have met

It might happen that CONFIG_GZIP and CONFIG_LZO are enabled but we might
have CONFIG_MULTI_DTB_FIT_LZO enabled in this case in the code path of
uncompress_blob() we parse the gzip headers first which results in
`Error: Bad gzipped data` being printed. To avoid this parse the gzip/lzo
headers only when dependencies have met.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Tom Rini <trini@konsulko.com>