n64.mk: fix installation directory
n64.mk: fix installation directory

n64.mk is supposed to be found via "include n64.mk" after installation.
This only works if it is installed in the user prefix, irrespective
of $N64_INST / $INSTALLDIR. This is the only file that must go outside
it.

sp1187/libdragon,sp1187/libdragon,DragonMinded/libdragon,DragonMinded/libdragon,DragonMinded/libdragon,sp1187/libdragon