Remove reference to COFF support. I can find no trace of ever having supported it.

src/config.h

 #undef	IMAGE_NBI		/* NBI image support */
 #undef	IMAGE_ELF64		/* ELF64 image support */
 #undef	IMAGE_ELF		/* ELF image support */
-#undef	IMAGE_COFF		/* COFF image support */
 #undef	IMAGE_FREEBSD		/* FreeBSD kernel image support */
 #define	IMAGE_MULTIBOOT		/* MultiBoot image support */
 #undef	IMAGE_AOUT		/* a.out image support */

src/core/config.c

 #ifdef IMAGE_ELF
 REQUIRE_OBJECT ( elf );
 #endif
-#ifdef IMAGE_ELF
-REQUIRE_OBJECT ( coff );
-#endif
 #ifdef IMAGE_FREEBSD
 REQUIRE_OBJECT ( freebsd );
 #endif