Use relative path to btsync.conf.doc in btsync-makeconfig.sh
Prepend ${srcdir} to PATH in build()

This allows btsync-makeconfig.sh to find the btsync executable in the
extracted sources dir even if it is not installed on the system.
Prepending means the extracted sources dir will be preferred if both
exist.
Fix btsync-makeconfig.sh referencing uninstalled files

During package build execution, btsync-makeconfig.sh assumed that the
file /usr/share/bittorrent-sync/btsync.conf.doc existed on the system,
and that 'btsync' existed on PATH. Basically it assumed that the package
was already installed, which certainly is not necessarily the case.

Fixes:
- Use a relative path to refer to btsync.conf.doc
- Prepend ${srcdir} to PATH so that the extracted 'btsync' executable
  does exist on PATH