recipetool: add appendfile subcommand

recipetool: add appendfile subcommand

Locating which recipe provides a file in an image that you want to
modify and then figuring out how to bbappend the recipe in order to
replace it can be a tedious process. Thus, add a new appendfile
subcommand to recipetool, providing the ability to create a bbappend
file to add/replace any file in the target system. Without the -r
option, it will search for the recipe packaging the specified file
(using pkgdata from previously built recipes). The bbappend will be
created at the appropriate path within the specified layer directory
(which may or may not be in your bblayers.conf) or if one already exists
it will be updated appropriately.

Fairly extensive oe-selftest tests are also provided.

Implements [YOCTO #6447].

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>