rPi: Update boot process for later Pi firmwares

* The bootprocess is better documented now.
  Put technical info in linker script, user info
  into info.txt

build/jam/board/raspberry_pi/BoardSetup

@@ -7,7 +7,7 @@ HAIKU_BOOT_PLATFORM = raspberrypi_arm ;
 # Various hardcoded addresses
 #
 
-HAIKU_BOARD_LOADER_BASE = 0x32000000 ;
+HAIKU_BOARD_LOADER_BASE = 0x0 ;
 
 #
 # Flash image

build/jam/board/raspberry_pi/info.txt

@@ -1,4 +1,5 @@
 Setting up the Haiku boot partition
+-------------------------------------
 
 partition 1 -- FAT32, bootable flag, type 'c'
 partition 2 -- BeFS, Haiku filesystem, type 'eb'
@@ -7,7 +8,7 @@ The boot partition must contain the following files to boot Haiku:
 * bootcode.bin : 2nd stage bootloader, starts with SDRAM disabled
 * loader.bin : 3rd stage bootloader, starts with SDRAM enabled
 * start.elf: The GPU binary firmware image, provided by the foundation.
-* kernel.img: Haiku Loader (haiku_loader)
+* haiku_loader: Haiku Loader
 * config.txt: A configuration file read by the GPU.
 
 Optional files:
@@ -20,4 +21,13 @@ Additional GPU firmware images, rename over start.elf to use them:
   - this is the default
 * arm224_start.elf : 224M ARM, 32M GPU split
   - (use this for Linux only with no 3D or video processing.
-    Its enough for the 1080p framebuffer, but not much else)
+	Enough for the 1080p framebuffer, but not much else)
+
+Building
+-------------------------------------
+jam -q -sHAIKU_BOOT_BOARD=raspberry_pi -sHAIKU_BOOT_PLATFORM=raspberrypi_arm haiku_loader
+
+config.txt options
+-------------------------------------
+kernel=haiku_loader
+disable_commandline_tags=1

src/system/boot/platform/raspberrypi_arm/entry.S

@@ -1,8 +1,14 @@
+.section .init
 .globl _start
 _start:
 	b jmp_loader
 
-.balign 0x8000, 0
+.section .text
+	/* Start "safe" code area.
+	 * 0x4000 minimum safe area
+	 * defined in boot_loader_raspberrypi_arm.ld
+	 */
+
 jmp_loader:
 	/* Start Haiku loader */
 

src/system/ldscripts/arm/boot_loader_raspberrypi_arm.ld

@@ -11,10 +11,22 @@ UND_STACK_SIZE	= 0;
 
 SECTIONS
 {
+	/*
+	 * Raspberry Pi boot logic:
+	 * start.elf loads the kernel specified in config.txt
+	 * if a dtb exists, it is loaded at 0x100 and the kernel at 0x8000
+	 * else if disable_commandline_tags true, set kernel load address to 0x0
+	 * else load kernel at 0x8000 and put atags at 0x100
+	 */
+
 	. = BOARD_LOADER_BASE;
 
-	/* text/read-only data */
-	.text :	{
+	/* .init is a stub that jumps to code at .text */
+	.init : {
+		*(.init)
+	}
+
+	.text 0x8000 : {
 		CREATE_OBJECT_SYMBOLS
 		*(.text .text.* .gnu.linkonce.t.*)
 		*(.plt)