stand: i386/amd64: Always use elf format for /boot/loader and pxeldr

The first level boot blocks have understood how to load ELF code since
1999. Switch /boot/loader and /boot/pxeldr over to being ELF format so
that in-tree tools can examine them more closely. In addition, one
could, in theory, now have a 'lo-mem' and a 'hi-mem' segment (though a
lot of work would need to be done with bounce buffers, btx, code segment
marking, etc for an arrangement like that to work).

As far as I can tell, this is the last a.out binary in the tree. There
are several raw binaries left, but everything else is ELF.

Reviewed by:		emaste, kevans
Sponsored by:		Netflix
Differential Revision:	https://reviews.freebsd.org/D36130

stand/i386/loader/Makefile

@@ -80,7 +80,7 @@ CFLAGS+=	-I${BOOTSRC}/i386
 	vtfontcvt -f compressed-source -o ${.TARGET} ${.ALLSRC}
 
 ${LOADER}: ${LOADER}.bin ${BTXLDR} ${BTXKERN}
-	btxld -v -f aout -e ${LOADER_ADDRESS} -o ${.TARGET} -l ${BTXLDR} \
+	btxld -v -f elf -e ${LOADER_ADDRESS} -o ${.TARGET} -l ${BTXLDR} \
 		-b ${BTXKERN} ${LOADER}.bin
 
 ${LOADER}.bin: ${LOADER}.sym

stand/i386/pxeldr/Makefile

@@ -39,7 +39,7 @@ LDFLAGS+=${LDFLAGS_BIN}
 CLEANFILES+= ${LOADER}
 
 ${LOADER}: ${LOADERBIN} ${BTXLDR} ${BTXKERN}
-	btxld -v -f aout -e ${LOADER_ADDRESS} -o ${.TARGET} -l ${BTXLDR} \
+	btxld -v -f elf -e ${LOADER_ADDRESS} -o ${.TARGET} -l ${BTXLDR} \
 	    -b ${BTXKERN} ${LOADERBIN}
 
 .include <bsd.prog.mk>