examples/baremetal: make paddr == vaddr

Fix loading of the example baremetal kernel with the fixed ELF kernel
loader:
- Remove the "RAM" address space so virtual and physical address match.
- Remove the 0x200-byte padding now that entry point address is used.

BUG=b:234155022
TEST=Run baremetal as in tools/examples/baremetal/README.md

Change-Id: I61394cdf4bf71f91736da5a636b0088ecfe78c84
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3691965
Commit-Queue: Daniel Verkamp <dverkamp@chromium.org>
Reviewed-by: Anton Romanov <romanton@google.com>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Tested-by: kokoro <noreply+kokoro@google.com>

tools/examples/baremetal/layout.ld

@@ -3,7 +3,6 @@ ENTRY(_start)
 /* Crosvm will load our code into 0x200000 + paddr */
 MEMORY {
     VMA : ORIGIN = 0x00200000, LENGTH = 0x200000
-    RAM : ORIGIN = 0x00000000, LENGTH = 0x200000
 }
 
 SECTIONS {
@@ -13,7 +12,7 @@ SECTIONS {
   {
       _stack_end = . ;
       *(.boot)
-  }> VMA AT>RAM
+  }> VMA
 
   .kernel :
   {
@@ -22,7 +21,7 @@ SECTIONS {
     *(.rodata .rodata.*)
     *(.data .data.*)
     *(.bss .bss.*)
-  }> VMA AT>RAM
+  }> VMA
 
   DISCARD/ :
   {

tools/examples/baremetal/src/boot.asm

@@ -1,12 +1,13 @@
+/* Copyright 2022 The ChromiumOS Authors.
+ * Use of this source code is governed by a BSD-style license that can be
+ * found in the LICENSE file.
+ */
+
 .section .boot, "awx"
 .global _start
 .code64
-# crosvm starts execution at 0x200 offset from the beginning
-.fill 0x200
 
 _start:
     lea rsp, _stack_end
 
     jmp main
-
-