mkimxboot: add a switch to force version

Add a switch to override the product and component version of the
sb file. This can usually for target like the Zen X-Fi2 where the
upader allows to drop any file named firmware.sb and prints the
version: by using a funky version the users can check they got it
right. This should not be used on the fuze+ or zenxfi3 because the
OF prevents downgrade.
Also make rbutil always zero out the option structure passed to
mkimxboot, this has already created bugs in the past.

Change-Id: I175c5def52c40c2132e11300e2f037d60a4f040e

rbutil/mkimxboot/mkimxboot.h

@@ -83,6 +83,7 @@ struct imx_option_t
     bool debug;
     enum imx_output_type_t output;
     enum imx_firmware_variant_t fw_variant;
+    const char *force_version; // set to NULL to ignore
 };
 
 void dump_imx_dev_info(const char *prefix);