combo-layer: improve patch list handling and output

combo-layer: improve patch list handling and output

* Ignore blank lines in patch list
* Don't fail in interactive mode if patch list is deleted
* Show patch counter
* Show relative path for patches
* Print headings before applying patch list for each component

Also change to using a "with" block to read the patch list so it gets
closed properly when we're finished.

Fixes [YOCTO #2455].

(From OE-Core rev: 65461d7c35fdadb5b008052798731dce19ed187f)

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