bios: Make sure PVH still works with "rom" feature

Without this change, QEMU/CloudHV will attempt to load the ROM into the
memory region below 4GiB. But something (i.e. SeaBIOS) is already there.

We just pick an arbitrary address. It doesn't actually matter where it
gets loaded, as the ROM code isn't used when doing PVH boot.

Signed-off-by: Joe Richey <joerichey@google.com>

layout.ld

@@ -1,15 +1,17 @@
 ENTRY(linux64_start)
 
+/* Loaders like to put stuff in low memory (< 1M), so we don't use it. */
+ram_min = 1M;
+/* ram32.s only maps the first 2 MiB, which must include our whole program. */
+ram_max = 2M;
+
 PHDRS
 {
   ram  PT_LOAD FILEHDR PHDRS ;
   note PT_NOTE               ;
-  rom  PT_LOAD               ;
+  rom  PT_LOAD AT(ram_max)   ;
 }
 
-/* Loaders like to put stuff in low memory (< 1M), so we don't use it. */
-ram_min = 1M;
-
 SECTIONS
 {
   /* Mapping the program headers and note into RAM makes the file smaller. */
@@ -37,8 +39,7 @@ SECTIONS
   /* Our stack grows down and is page-aligned. TODO: Add stack guard pages. */
   .stack (NOLOAD) : ALIGN(4K) { . += 64K; }
   stack_start = .;
-  /* ram32.s only maps the first 2 MiB, and that must include the stack. */
-  ASSERT((. <= 2M), "Stack overflows initial identity-mapped memory region")
+  ASSERT((. <= ram_max), "Stack overflows initial identity-mapped region")
 
   /* This is correct because all of the code sections have alignment 16. */
   rom_size = ALIGN(SIZEOF(.gdt32), 16) + ALIGN(SIZEOF(.rom32), 16)