We cannot jump to SDRAM unconditionally in imx*_barebox_boot_nand_external.
When we really boot from NOR flash the binary is not yet copied to SDRAM.
Instead, let the return value of imx_barebox_boot_nand_external() indicate
whether we really boot from NAND and only jump to SDRAM in this case.
Otherwise just continue to the normal SoC specific entry.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>