vsftpd: Add method for providing custom initscript
vsftpd: Add method for providing custom initscript

To use a custom init script, add a recipe that builds a package that
provides "vsftpd-init" and set it's DEFAULT_PREFERENCE to something
higher than 0.

Signed-off-by: Esben Haabendal <esben.haabendal@prevas.dk>

(cherry picked from commit 46f4db194a3a5a0553fe34b83216fc7c91eeacd7)

jacobbarsoe/rpi-base,jacobbarsoe/rpi-base,oe-lite-rpi/base,oe-lite-rpi/base