otasigcheck: Extract the key

* commit b110c751
  "build: ota: Support for install tools in /tmp/install"
  erroneously removed the line extracting the releasekey,
  making the script a no-op as it couldn't find a key to
  compare against.

Change-Id: I0dc5d15dbf4b0531de4df9e62a5bd47ec463c2a1