envsetup: Mark zsh as compatible

Basic functionality has been tested on zsh 5.0.2 and has been found to
work correctly.

Change-Id: I02e5e3bedf56b43104c280d9737ae7b334357643

Conflicts:

	envsetup.sh
core/Makefile: update BOARD_CUSTOM_BOOTIMG_MK fix
this updates code left out from commit fd88ca3

Change-Id: I86ceb22001b92f1b631000d0cefa84a64d11370e