build: fixup python SetupHostCommand to use python2

Here is a way to break your build env without this patch:
1) have python point to python3, and no python2
2) start the build, SetupHostCommand will create a symlink
./staging_dir/host/bin/python -> /usr/bin/python
3) build fails on scons because it can't find any python2
4) install python2 and restart the build
5) the build fails on wireless-regdb compile because python is python3 instead of python

Signed-off-by: Etienne Champetier <champetier.etienne@gmail.com>

include/prereq-build.mk

@@ -142,9 +142,9 @@ $(eval $(call SetupHostCommand,perl,Please install Perl 5.x, \
 	perl --version | grep "perl.*v5"))
 
 $(eval $(call SetupHostCommand,python,Please install Python 2.x, \
-	python2.7 -V 2>&1 | grep Python, \
-	python2 -V 2>&1 | grep Python, \
-	python -V 2>&1 | grep Python))
+	python2.7 -V 2>&1 | grep 'Python 2.7', \
+	python2 -V 2>&1 | grep 'Python 2', \
+	python -V 2>&1 | grep 'Python 2'))
 
 $(eval $(call SetupHostCommand,git,Please install Git (git-core) >= 1.7.12.2, \
 	git --exec-path | xargs -I % -- grep -q -- --recursive %/git-submodule))