make: "make flash" re-builds the HEXFILE if necessary

 * The HEXFILE depends on the ELFFILE.
 * The ELFFILE depends on the object files (UNDEF and BASELIBS).
 * If the source files did change, "make flash" will automatically re-compile and link before flashing.
 * If the source files did not change, "make flash" will not re-compile or link before flashing.
 * "make compile" and "make link" are also available.
 * The behavior of "make" / "make all" still depends on RIOTNOLINK.
 * Parallel builds (make -j5) seem to work fine.