add option to hide TBB's logo

Speaking as a user of Tails and a maintainer of Whonix, I want to point out, that showing the Tor Browser Bundle Logo when using tor-launcher only for configuring Tor without TBB is inappropriate and confusing. It should not be shown in that mode.

Could you please add either:

• an environment variable such as TOR_HIDE_LOGO, or
• maybe it is enough to omit src/chrome/skin/tbb-logo.png when creating the standalone target using the Makefile