Rpi: Convert start.c to cpp

* Wrap _start in extern "C" to prevent
  symbol mangling.
* Thanks mmlr for reminding me of this! :)

src/system/boot/platform/raspberrypi_arm/Jamfile

@@ -22,7 +22,7 @@ SEARCH_SOURCE += [ FDirName $(HAIKU_TOP) src add-ons accelerants common ] ;
 
 BootMergeObject boot_platform_raspberrypi_arm.o :
 	entry.S
-	start.c
+	start.cpp
 	console.cpp
 	cpu.cpp
 	debug.cpp

src/system/boot/platform/raspberrypi_arm/start.cpp

@@ -15,7 +15,7 @@
 #include <KernelExport.h>
 #include <boot/platform.h>
 #include <boot/heap.h>
-//#include <boot/stage2.h>
+#include <boot/stage2.h>
 #include <arch/cpu.h>
 
 #include <string.h>
@@ -102,7 +102,7 @@ platform_exit(void)
 }
 
 
-void
+extern "C" void
 _start(void)
 {
 	stage2_args args;
@@ -128,4 +128,3 @@ _start(void)
 
 	main(&args);
 }
-