tools/build: Introduce {PREPEND,APPEND}_{LIB,INCLUDES}

tools/build: Introduce {PREPEND,APPEND}_{LIB,INCLUDES}

Create two new variables called APPEND_ and PREPEND_ to add compile
flags at the beginning or at the end of the search path.

Added a new semantic for user defined compile flags, here is the list
of possible options:

PREPEND_LIB: add libraries to the search path before xen
             (before xen installation folders).
PREPEND_INCLUDES: add headers to the search path before xen
                  (before xen installation folders).
APPEND_LIB: add libraries to the search path at the end
            (after all xen installation folders have been added).
APPEND_INCLUDES: add libraries to the search path at the end
                 (after all xen installation folders have been added).

EXTRA_INCLUDES and EXTRA_LIB can still be used, and they will have the
same effect as PREPEND_INCLUDES and PREPEND_LIB.

Signed-off-by: Roger Pau Monne <roger.pau@entel.upc.edu>
Committed-by: Ian Jackson <ian.jackson@eu.citrix.com>