envsetup: fix clang path for kernel building

Summary: Without this patch kernel.mk fails at finding clang,thus reporting No such file or directory

Test Plan:
Set TARGET_ARM_EABI_VERSION to clang or clang-3.4
make bootimage

Reviewers: synergy, randall.rushing

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D648

Change-Id: I557be90d3cebacad3b90c6f4f16875edf827c1e5