Use RGBDS 0.4.0 and make boilerplate minimal

src/header.asm

@@ -0,0 +1,21 @@
+
+SECTION "Header", ROM0[$100]
+
+	; This is your ROM's entry point
+	; You have 4 bytes of code to do... something
+	di
+	jp EntryPoint
+
+	; Make sure to allocate some space for the header, so no important
+	; code gets put there and later overwritten by RGBFIX.
+	; RGBFIX is designed to operate over a zero-filled header, so make
+	; sure to put zeros regardless of the padding value. (This feature
+	; was introduced in RGBDS 0.4.0, but the -MG etc flags were also
+	; introduced in that version.)
+	ds $150 - @, 0
+
+SECTION "Entry point", ROM0
+
+EntryPoint:
+	; Here is where the fun begins, happy coding :)
+	jr @