Support for multiple BR2_GLOBAL_PATCH_DIR

Adding support for specifying multiple directories in
BR2_GLOBAL_PATCH_DIR. This will allow for a layered approach for the
patching of a package.

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>

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Changes v2 -> v3:
  - changed the generation of patch directories to use 'addsuffix'
    instead of a foreach loop. (suggested by Arnout)

Changes v1 -> v2:
  - change wording in Config.in help (suggested by Thomas D)
manual: update for multiple global patch dirs

Updating the documentation to reflect that multiple directories can
now be specified for BR2_GLOBAL_PATCH_DIR. Along with giving an
example use case of how to use multiple global patch directories.

Signed-off-by: Ryan Barnett <rjbarnet@rockwellcollins.com>

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Changes v2 -> v3:
  - None

Changes v1 -> v2:
  - Fixed minor spelling mistakes and wording (suggested by Thomas D)