main/dtbtool-sprd: new aport

dtbtool-sprd is a version of dtbtool which is meant for Spreadtrum devices. Spreadtrum uses different header magic, so when dtbtool is used during kernel build, it can't find any valid dtb (known No valid dtbs found error). The hack mentioned at the wiki page isn't needed anymore.

Tested with upcoming device which wouldn't boot with the hack. Works fine.