Multiple boards support: clone once, build multiple images for different boards.
1. split clone to clone and build
    --clone will ONLY get source code
    --build will get .config and build firmware
2. add AVA_TARGET_BOARD environment variable for build different boards
    use TP-Link WR703n-v1 if the variable is unset
    support WR703n-v1, WR1043ND-v2, Raspberry-Pi now
3. more update according to OpenWrt HowTo
4. output path changed, add board-name
5. replace TAB to spaces
6. Update version to 20140415, because of the significant change