main/blobtools: Allow blob signing

Certain recent Asus Tegra SoC based devices (tf201, tf300, tf700) only accept signed blobs, this was implemented upstream but seems that some lines got deleted so it wasn't being signed which was fine for devices (Asus tf101) that didn't require it.

To reuse the existing setup I added a patch for blobtools that let us enable singing via optional flag, this same flag is only enabled when device has deviceinfo_bootimg_blobpack="sign" existing devices (tf101) use deviceinfo_bootimg_blobpack="true" which shouldn't be affected by these changes.

After this MR there is plans to send MR for initial asus-tf201 bringup.