Currently, *.bb and *.bbclass files are automatically highlighted and
detected as a bitbake file. Since *.bbappend files are also bitbake
files, vim plugin should automatically detect and highlight it as well.
(Bitbake rev: 0668a0ea968baf930f05806a5452c8dbe4ec35ce)
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>