Add wrapper scripts for OS operations and allow disabling git pull

- Add support for a set of wrapper scripts which are used to start/stop
  fppd, git pull, etc.

- Add the ability to disable the git auto update at boot time.  On
  the about page, there is a 'Disable Auto Update' checkbox which
  can be used to disable auto update.

- Add a 'developer mode' flag.  Currently this is only used to enable
  an option to allow changing git branches via the UI on the about page.

The wrapper scripts added are:

scripts/common         - common functions for scripts
scripts/fpp_boot       - boot script run from /etc/rc.local
scripts/fpp_build      - compile script to build fpp/fppd binaries
scripts/fppd_start     - fppd startup script
scripts/fppd_stop      - fppd stop script
scripts/git_branch     - helper for switching git branches
scripts/git_pull       - helper for running a 'git pull'
scripts/upgrade_config - OS configuration wrapper script

The upgrade_config script runs config files in the upgrades/X
directories where 'X' is a version number.  The first version 1
is used to modify the /etc/rc.local file to convert an installed
system to use the new fpp_boot script instead of hard-coding
the git pull and fppd start commands in the /etc/rc.local script.

To add a new config change to update something in the base OS
image, just create a new 'X' version directory (counting upwards
by 1 each time) and create an 'upgrade.sh' script inside this
directory.  The upgrade.sh scripts are executed in order and
only if necessary.  The current config version is tracked in
/etc/fpp/config_version.
Add new fpp and fppd binaries to go along with the multitude of commits
in this branch to make testing easier.