Rename bootloader to main

The presence of the main function is a hardcoded trigger in sdcc that causes
the interrupt vectors and atomic rollback helpers to be generated as part of
compilation of the file containing the main function.

This makes sure the runtime library is correctly initialized and reduces
crtstart.asm to contain only the banked call helpers.

To ensure that the __interrupt_vect in area HOME is located at the beginning
of the firmware, the linker flags are updated to set the HOME start to
0x00000 and the linker object order is modified to have rtlplayground.rel
first.

Generated code in rtlplayground.asm:
```
;--------------------------------------------------------
; interrupt vector
;--------------------------------------------------------
	.area HOME    (CODE)
__interrupt_vect:
	ljmp	__sdcc_gsinit_startup
	ljmp	_isr_ext0
	.ds	5
	ljmp	_isr_timer0
	.ds	5
	ljmp	_isr_ext1
	.ds	5
	reti
	.ds	7
	ljmp	_isr_serial
	.ds	5
	ljmp	_isr_timer2
	.ds	5
	reti
	.ds	7
	reti
	.ds	7
	ljmp	_isr_ext2
	.ds	5
	ljmp	_isr_ext3
; restartable atomic support routines
	.ds	2
sdcc_atomic_exchange_rollback_start::
	nop
	nop
sdcc_atomic_exchange_pdata_impl:
	movx	a, @r0
	mov	r3, a
	mov	a, r2
	movx	@r0, a
	sjmp	sdcc_atomic_exchange_exit
	nop
	nop
sdcc_atomic_exchange_xdata_impl:
	movx	a, @dptr
	mov	r3, a
	mov	a, r2
	movx	@dptr, a
	sjmp	sdcc_atomic_exchange_exit
sdcc_atomic_compare_exchange_idata_impl:
	mov	a, @r0
	cjne	a, ar2, .+#5
	mov	a, r3
	mov	@r0, a
	ret
	nop
sdcc_atomic_compare_exchange_pdata_impl:
	movx	a, @r0
	cjne	a, ar2, .+#5
	mov	a, r3
	movx	@r0, a
	ret
	nop
sdcc_atomic_compare_exchange_xdata_impl:
	movx	a, @dptr
	cjne	a, ar2, .+#5
	mov	a, r3
	movx	@dptr, a
	ret
sdcc_atomic_exchange_rollback_end::

sdcc_atomic_exchange_gptr_impl::
	jnb	b.6, sdcc_atomic_exchange_xdata_impl
	mov	r0, dpl
	jb	b.5, sdcc_atomic_exchange_pdata_impl
sdcc_atomic_exchange_idata_impl:
	mov	a, r2
	xch	a, @r0
	mov	dpl, a
	ret
sdcc_atomic_exchange_exit:
	mov	dpl, r3
	ret
sdcc_atomic_compare_exchange_gptr_impl::
	jnb	b.6, sdcc_atomic_compare_exchange_xdata_impl
	mov	r0, dpl
	jb	b.5, sdcc_atomic_compare_exchange_pdata_impl
	sjmp	sdcc_atomic_compare_exchange_idata_impl
```

Makefile

@@ -1,5 +1,3 @@
-BOOTLOADER_ADDRESS=0x100
-
 VERSION=0.1.0
 IMAGESIZE = 524288
 DEFAULT_CONFIG_LOCATION = 454656
@@ -65,8 +63,8 @@ $(BUILDDIR)%.rel: $(BUILDDIR)%.asm
 	${ASM} ${AFLAGS} -o $@ $<
 #	mv -f $(addprefix $(basename $^), .lst .rel .sym) .
 
-$(BUILDDIR)rtlplayground.ihx: $(BUILDDIR)crtstart.rel $(OBJS) $(BUILDDIR)crc16.rel
-	$(CC) $(CC_FLAGS) -Wl-bHOME=${BOOTLOADER_ADDRESS} -Wl-bBANK1=0x14000 -Wl-bBANK2=0x24000 -Wl-r -o $@ $^
+$(BUILDDIR)rtlplayground.ihx: $(OBJS) $(BUILDDIR)crtstart.rel $(BUILDDIR)crc16.rel
+	$(CC) $(CC_FLAGS) -Wl-bHOME=0x00000 -Wl-bBANK1=0x14000 -Wl-bBANK2=0x24000 -Wl-r -o $@ $^
 
 $(BUILDDIR)rtlplayground.img: $(BUILDDIR)rtlplayground.ihx
 	objcopy --input-target=ihex -O binary $< $@