imximage: move flash_offset check to common location

Both set_imx_hdr_v1 and set_imx_hdr_v2 perform the
same check. Move check to before the set_imx_hdr call.

Signed-off-by: Troy Kisky <troy.kisky@boundarydevices.com>
Acked-by: Stefano Babic <sbabic@denx.de>