Don't check the host JDK tools when build/core/config.mk is called from envsetup.sh.

Envsetup.sh (lunch) relies on config.mk to get build varaible values; while
config.mk may rely on envsetup.sh to set up JAVA_HOME/PATH.
Without this change config.mk may be checking the host JDK tools before
JAVA_HOME/PATH get set up.

Change-Id: I1e6b5e33147258832ce2f0a425f9526ef4782c43