Makefile: Add build rules for .bin and .tap files.

Pasmo is used for the assembler.

Issuing 'make tap' will sna2skool, skool2asm, pasmo then bin2tap a
runnable game image.
Makefile: Rewrite rules so that we don't rebuild needlessly.
Makefile: Make the rules generic where possible.
Makefile: Add an 'all' rule which builds everything.
zoombox_1 -> zoombox_fill (to avoid generated label clash).
Add labels for the benefit of generated .asm.
exterior_mask_data -> outdoors_mask (to avoid name clash).
Add data labels for the benefit of generated .asm.
Add roomdef labels for the benefit of generated .asm.
Add further data labels for the benefit of generated .asm.