Move 32bit segment defines from romlayout.S to config.h.

This moves the 32bit segment names next to the real-mode segment names.
Also, renames them to be more consistent.
diff --git a/src/config.h b/src/config.h
index 39a796d..354adcf 100644
--- a/src/config.h
+++ b/src/config.h
@@ -95,11 +95,17 @@
 // Start of fixed addresses in 0xf0000 segment.
 #define BUILD_START_FIXED       0xe050
 
-// Important 16-bit segments
+// Important real-mode segments
 #define SEG_BIOS     0xf000
 #define SEG_EBDA     0x9fc0
 #define SEG_BDA      0x0000
 
+// Segment definitions in 32bit mode.
+#define SEG32_MODE32_CS (2 << 3) // 0x10
+#define SEG32_MODE32_DS (3 << 3) // 0x18
+#define SEG32_MODE16_CS (4 << 3) // 0x20
+#define SEG32_MODE16_DS (5 << 3) // 0x28
+
 // Debugging levels.  If non-zero and CONFIG_DEBUG_LEVEL is greater
 // than the specified value, then the corresponding irq handler will
 // report every enter event.
diff --git a/src/romlayout.S b/src/romlayout.S
index dabee9a..e9ea067 100644
--- a/src/romlayout.S
+++ b/src/romlayout.S
@@ -7,11 +7,6 @@
 
 #include "config.h"
 
-#define PROTECTED_MODE_CS (2 << 3) // 0x10
-#define PROTECTED_MODE_DS (3 << 3) // 0x18
-#define REAL_MODE_CS      (4 << 3) // 0x20
-#define REAL_MODE_DS      (5 << 3) // 0x28
-
 
 /****************************************************************
  * Include of 16bit C code
@@ -164,12 +159,12 @@
         movl  %eax, %cr0
 
         // start protected mode code
-        ljmpl $PROTECTED_MODE_CS, $(BUILD_BIOS_ADDR + 1f)
+        ljmpl $SEG32_MODE32_CS, $(BUILD_BIOS_ADDR + 1f)
 
         .code32
 1:
         // init data segments
-        movl $PROTECTED_MODE_DS, %eax
+        movl $SEG32_MODE32_DS, %eax
         movw %ax, %ds
         movw %ax, %es
         movw %ax, %ss
@@ -187,12 +182,12 @@
         pushl %eax
 
         // Jump to 16bit mode
-        ljmpw $REAL_MODE_CS, $1f
+        ljmpw $SEG32_MODE16_CS, $1f
 
         .code16gcc
 1:
         // restore data segment limits to 0xffff
-        movw $REAL_MODE_DS, %ax
+        movw $SEG32_MODE16_DS, %ax
         movw %ax, %ds
         movw %ax, %es
         movw %ax, %ss
@@ -319,7 +314,7 @@
         lidtl (BUILD_BIOS_ADDR + pmode_IDT_info)
         lgdtl (BUILD_BIOS_ADDR + rombios32_gdt_48)
         movl $BUILD_STACK_ADDR, %esp
-        ljmpl $PROTECTED_MODE_CS, $_code32__start
+        ljmpl $SEG32_MODE32_CS, $_code32__start
 
         .code16gcc
 
@@ -369,13 +364,13 @@
 rombios32_gdt:
         .word 0, 0, 0, 0
         .word 0, 0, 0, 0
-        // 32 bit flat code segment (PROTECTED_MODE_CS)
+        // 32 bit flat code segment (SEG32_MODE32_CS)
         .word 0xffff, 0, 0x9b00, 0x00cf
-        // 32 bit flat data segment (PROTECTED_MODE_DS)
+        // 32 bit flat data segment (SEG32_MODE32_DS)
         .word 0xffff, 0, 0x9300, 0x00cf
-        // 16 bit code segment base=0xf0000 limit=0xffff (REAL_MODE_CS)
+        // 16 bit code segment base=0xf0000 limit=0xffff (SEG32_MODE16_CS)
         .word 0xffff, 0, 0x9b0f, 0x0000
-        // 16 bit data segment base=0x0 limit=0xffff (REAL_MODE_DS)
+        // 16 bit data segment base=0x0 limit=0xffff (SEG32_MODE16_DS)
         .word 0xffff, 0, 0x9300, 0x0000
 
 // We need a copy of this string in the 0xf000 segment, but we are not