build: Move patches to <patch-repos> dir

build: Move patches to <patch-repos> dir

Decouple main Makefile and repository patches by moving all Fuel
component patches to a subdir, <patch-repo>.

Proposed structure of <patch-repos> dir:
- patch-repos/*.patch are intended for <fuel-main> git repo;
- patch-repos/build/repo/<fuel-module>/*.patch are intended for
  <fuel-module>, e.g. "fuel-nailgun";

Full list of Fuel submodules supported by this mechanism at [1].

While at it, replace all occurences of </tmp/fuel-main> with
<FUEL_MAIN_DIR> in main Makefile and rewrite a few git/make calls
to use the "-C" arg.

NOTE: `git -C` is available starting with git 1.8.5.

[1] https://github.com/openstack/fuel-main/blob/stable/mitaka/repos.mk
    #L32-L44

Change-Id: I5e4adc7285054777c3898592ed48482ccd2b706f
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>