Add PPC to U-Boot bootloader support.

This should be useful for SAM440 and SAM460 boards for example.
The generic U-Boot code still has some ARM-specific parts that must be moved out though.

src/system/boot/platform/u-boot/arch/ppc/Jamfile

@@ -0,0 +1,19 @@
+SubDir HAIKU_TOP src system boot platform openfirmware arch ppc ;
+
+SubDirHdrs $(HAIKU_TOP) src system boot platform $(TARGET_BOOT_PLATFORM) ;
+UsePrivateSystemHeaders ;
+UsePrivateHeaders kernel [ FDirName kernel arch $(TARGET_ARCH) ] 
+	[ FDirName kernel boot platform $(HAIKU_BOOT_PLATFORM) ] ;
+
+SubDirC++Flags -fno-rtti ;
+
+KernelStaticLibrary boot_platform_openfirmware_ppc :
+	arch_mmu.cpp
+	arch_cpu_asm.S
+	arch_start_kernel.S
+	cpu.cpp
+	mmu.cpp
+;
+
+SEARCH on [ FGristFiles arch_cpu_asm.S arch_mmu.cpp ]
+    = [ FDirName $(HAIKU_TOP) src system kernel arch $(TARGET_ARCH) ] ;

src/system/boot/platform/u-boot/arch/ppc/arch_start_kernel.S

@@ -0,0 +1,49 @@
+/*
+ * Copyright 2005, Ingo Weinhold <bonefish@cs.tu-berlin.de>.
+ * All rights reserved. Distributed under the terms of the MIT License.
+ */
+
+#define FUNCTION(x) .global x; .type x,@function; x
+
+/*	status_t arch_start_kernel(struct kernel_args *kernelArgs,
+		addr_t kernelEntry, addr_t kernelStackTop);
+
+	r3	- kernelArgs
+	r4	- kernelEntry
+	r5	- kernelStackTop
+*/
+FUNCTION(arch_start_kernel):
+	// push a stack frame
+	stwu	%r1, -32(%r1)
+	mflr	%r0
+	stw		%r0, 36(%r1)
+
+	// save the old stack pointer in r29
+	stw		%r29, 20(%r1)
+	mr		%r29, %r1
+
+	// set up the kernel stack
+	subi	%r1, %r5, 16
+
+	// clear the pointer to the previous frame
+	li		%r0, 0
+	stw		%r0, 0(%r1)
+
+	// enter the kernel
+	mtlr	%r4
+	li		%r4, 0
+	blrl
+
+	/* Actually we should never get here, but at least for debugging purposes
+	   it's quite nice to return in an orderly manner. */
+
+	// reset the boot loader stack
+	mr		%r1, %r29
+	lwz		%r29, 20(%r1)
+
+	// pop the stack frame
+	lwz		%r0, 36(%r1)
+	mtlr	%r0
+	addi	%r1, %r1, 32
+	blr
+