Modernize some RGBASM syntax to silence most deprecation warnings in the upcoming 0.7 RGBDS release

BootROMs/agb_boot.asm

@@ -1,2 +1,2 @@
-AGB EQU 1
+DEF AGB = 1
 include "cgb_boot.asm"

BootROMs/cgb0_boot.asm

@@ -1,2 +1,2 @@
-CGB0 EQU 1
+DEF CGB0 = 1
 include "cgb_boot.asm"

BootROMs/cgb_boot.asm

@@ -665,8 +665,8 @@ ReadCGBLogoHalfTile:
 ; LoadTileset using PB12 codec, 2020 Jakub Kądziołka
 ; (based on PB8 codec, 2019 Damian Yerrick)
 
-SameBoyLogo_dst = $8080
-SameBoyLogo_length = (128 * 24) / 64
+DEF SameBoyLogo_dst = $8080
+DEF SameBoyLogo_length = (128 * 24) / 64
 
 LoadTileset:
     ld hl, SameBoyLogo

BootROMs/cgb_boot_fast.asm

@@ -1,2 +1,2 @@
-FAST EQU 1
+DEF FAST = 1
 include "cgb_boot.asm"

BootROMs/mgb_boot.asm

@@ -1,2 +1,2 @@
-MGB EQU 1
+DEF MGB = 1
 include "dmg_boot.asm"

BootROMs/sgb2_boot.asm

@@ -1,2 +1,2 @@
-SGB2 EQU 1
+DEF SGB2 = 1
 include "sgb_boot.asm"