mk: support renamed Makefile in external project

mk: support renamed Makefile in external project

The build system made a recursive call to "make" after
creating the build directory. This recursive call used
the hard-coded filename "Makefile", which prevented
builds from working if the file was renamed and make
called using "make -f". Taking the filename from
MAKEFILES_LIST make variable fixes this.

Fixes: af75078fece3 ("first public release")
Cc: stable@dpdk.org
Signed-off-by: Marko Kovacevic <marko.kovacevic@intel.com>
Acked-by: Vipin Varghese <vipin.varghese@intel.com>