make-release: drop rootDir from makeTarFile() calls

Some left over from a previous refactoring I guess.

Change-Id: I841b2cfe2ca1f0f5a3df7fd4ea6a95be3e79184a
make-release: nicer exit when gpg fails/is absent

Change-Id: I49729c9c203168ad42cf2dca6c0f615ede0483f8