Add portable utilities for creating tiles, add AmigaFont to symbols

- bmp2iff_host: convert nhtiles.bmp to Amiga IFF tile files. Uses
  the AMIV UI palette in pens 0-15, remaining pens filled with tile
  colors sorted by frequency.
  Usage: bmp2iff_host -planes N input.bmp output.iff

- xpm2iff_host: convert XPM to IFF for tomb.iff (RIP screen).
  Adapted from xpm2iff.c, Copyright (c) 1995 Gregg Wonderly.

- Auto-select tiles32.iff (5 planes) or tiles16.iff (4 planes)
  based on screen color depth at runtime.

- Fix NO_GLYPH in amiv_lprint_glyph: return early to prevent
  blitting with uninitialised data (caused black spots).

- Add AmigaFont symbol set to dat/symbols for AMII text mode.

sys/amiga/README.amiga

@@ -73,8 +73,8 @@ Cross-compilation from Linux using bebbo's m68k-amigaos-gcc toolchain
     # Install toolchain to /opt/amiga
     # Clone NetHack 3.7 source
     cd NetHack
-    make fetch-lua
     sys/unix/setup.sh sys/unix/hints/linux.370
+    make fetch-lua
     make CROSS_TO_AMIGA=1 all
     make CROSS_TO_AMIGA=1 package