Renamed .buildgear/.lock to .buildgear/lock

No need to make the lock file hidden since it is already in a hidden
directory.
Add rebuild detection based on buildfile checksum

This means that a build will be automatically rebuilt if the sha256sum
checksum of its bash expanded Buildfile changes.

This is particular useful if one uses configurations variables to
reconfigure the way your firmware is built.

For example, if you have a configuration variable in the global
buildfiles configuration file (config/config), eg.:

CONFIG_AUDIO="alsa"

And in one or more Buildfiles you use this variable to reconfigure the
build or change dependencies. For example:

if [ "$CONFIG_AUDIO" == "alsa" ]
then
   CONF_OPTIONS+="--audio=alsa"
   depends+=(alsa-lib)
else
   CONF_OPTIONS+="--audio=oss"
fi

Then, if you change CONFIG_AUDIO then build gear will automatically
rebuild any buildfiles that uses CONFIG_AUDIO because it will cause the
checksum of the bash expanded buildfile(s) to change.