src/lib/prog_loaders: Add preload_ramstage

This will enable preloading ramstage. By preloading the file into
cbfs_cache we reduce boot time.

BUG=b:179699789
TEST=Boot guybrush to OS and see 12ms reduction in boot time.

Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Ibe12de806449da25bc0033b02fcb97c3384eddc1
Reviewed-on: https://review.coreboot.org/c/coreboot/+/58982
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

src/include/program_loading.h

@@ -145,6 +145,13 @@ int legacy_romstage_select_and_load(struct prog *romstage);
  *   RAMSTAGE LOADING   *
  ************************/
 
+/*
+ * Asynchronously preloads ramstage.
+ *
+ * This should be called early on to allow ramstage to load before
+ * `run_ramstage` is called.
+ */
+void preload_ramstage(void);
 /* Run ramstage from romstage. */
 void run_ramstage(void);