imximage: Remove overwriting of flash_offset

The flash header supports different flash offsets for different
boot devices. E.g. parallel NOR or OneNAND use a different offset
than FLASH_OFFSET_STANDARD (== 0x400).

The flash offset is correctly read from the configuration in
parse_cfg_cmd(). But is then overwritten wrongly in set_imx_hdr_v1/2().

Fix this by removing this overwriting. Use the flash offset
correctly read from the configuration, instead.

If there is no flash_offset read from the configuration file, i.e.
the BOOT_FROM tag is missing, exit with an error message.

Signed-off-by: Dirk Behme <dirk.behme@de.bosch.com>
CC: Jason Liu <liu.h.jason@gmail.com>
CC: Stefano Babic <sbabic@denx.de>
Tested-by: Stefano Babic <sbabic@denx.de>