Fix build.sh that can't fetch FW_REPO branches and some minor fixes

[gnought]

• Renamed FW_REPO from Re4son/RPi-Distro-firmware to Re4son/kalipi-firmware
• Changed FIRMWARE_DIR to a new dir as FW repo changed
• Cloned FW_REPO by a treeless clone, not shadow clone. This will fix git checkout $FW_BRANCH in pull_firmware() function which gets expected branch.
• Created PKG_IN if it didn't exist