x86/EFI: Fix detection of buildid

The format of the buildid is a property of the binary, not a property of how
it was loaded.  This fixes buildid recognition when starting xen.efi from it's
MB2 entrypoint.

Suggested-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Frediano Ziglio <frediano.ziglio@cloud.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Acked-by: Daniel P. Smith <dpsmith@apertussolutions.com>

xen/common/version.c

@@ -203,8 +203,11 @@ void __init xen_build_init(void)
     rc = xen_build_id_check(n, sz, &build_id_p, &build_id_len);
 
 #ifdef CONFIG_X86
-    /* Alternatively we may have a CodeView record from an EFI build. */
-    if ( rc && efi_enabled(EFI_LOADER) )
+    /*
+     * xen.efi built with a new enough toolchain will have a CodeView record,
+     * not an ELF note.
+     */
+    if ( rc )
     {
         const struct pe_external_debug_directory *dir = (const void *)n;