tools: imximage: add plugin support

Add plugin support for imximage.

Define CONFIG_USE_IMXIMG_PLUGIN in defconfig to enable using plugin.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Eric Nelson <eric@nelint.com>
Cc: Ye Li <ye.li@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>