m68k: add next_m68k boot platform

Since the boot ROM API structure is declared with a custom alignment, we
simulate it with padding bytes around, and use aligntest.prg to verify
using ARAnyM + TOS/MiNT as we know how to link simple PRG files.

It now prints something to the screen then panics when initializing the
heap.

For now one must insert the loader manually into an existing floppy
image:
dd if=generated-m68k/objects/haiku/m68k/release/system/boot/next_m68k/haiku_loader.next_m68k bs=$((0x8000)) seek=1 of=next_floppy.img conv=notrunc

Change-Id: I06d74e9d85a352aab68dedce545bbe5fe9e990d5
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2220
Reviewed-by: Adrien Destugues <[email protected]>

build/jam/ArchitectureRules

@@ -314,7 +314,7 @@ rule KernelArchitectureSetup architecture
 
 		case m68k :
 			HAIKU_KERNEL_PLATFORM ?= atari_m68k ;
-			HAIKU_BOOT_TARGETS += amiga_m68k atari_m68k ;
+			HAIKU_BOOT_TARGETS += amiga_m68k atari_m68k next_m68k ;
 			switch $(HAIKU_KERNEL_PLATFORM) {
 				case atari_m68k :
 				{
@@ -325,6 +325,10 @@ rule KernelArchitectureSetup architecture
 					# for now we have trouble reading from double-sided images
 					HAIKU_BOOT_FLOPPY_IMAGE_SIZE = 880 ; # in kB
 				}
+				case next_m68k :
+				{
+					HAIKU_BOOT_FLOPPY_IMAGE_SIZE = 1440 ; # in kB
+				}
 			}
 			# offset in floppy image (>= sizeof(haiku_loader))
 			HAIKU_BOOT_ARCHIVE_IMAGE_OFFSET = 260 ; # in kB