more automation in Makefile

Hi 
I am hoping following changes in main Makefile can be accepted.
Goal is to use tight bootstrap.sh and source dev.env into a check: target.
als make without option is to show a help menu instead of going into build action.
```
-bash-4.1$ make
Please use `make <target>' where <target> is one of
  build   to build vitess Go code
  test    to test using python and Java test suite
  test1   to test only unit_test
  test2   to test unit_test and query_intergration_test
  check   to check build environment
  clean   to clean the build environment
  clean-mysqld to kill outstanding mysqld process
  <target> -n to see the No action make
-bash-4.1$ make clean -n && make build -n
go clean -i ./go/...
rm -rf java/vtocc-client/target java/vtocc-jdbc-driver/target third_party/acolyte
./bootstrap.sh
echo
echo "End of boot: ./bootstrap.sh"
(./bootstrap.sh && source ./dev.env)	
echo
echo "End of check: source ./dev.env"
(./bootstrap.sh && source ./dev.env && go install ./go/...)
echo
echo "End of build: go install ./go/..."
-bash-4.1$ 
```