cbfs: Move more stuff into cbfs_boot_lookup()

cbfs_boot_locate() is supposed to be deprecated eventually, after slowly
migrating all APIs to bypass it. That means common features (like
RO-fallback or measurement) need to be moved to the new
cbfs_boot_lookup().

Also export the function externally. Since it is a low-level API and
most code should use the higher-level loading or mapping functions
instead, put it into a new <cbfs_private.h> to raise the mental barrier
for using this API (this will make more sense once cbfs_boot_locate() is
removed from <cbfs.h>).

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I4bc9b7cbc42a4211d806a3e3389abab7f589a25a
Reviewed-on: https://review.coreboot.org/c/coreboot/+/39327
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>

src/include/cbfs.h

@@ -42,6 +42,12 @@ size_t cbfs_load_and_decompress(const struct region_device *rdev, size_t offset,
 /* Load stage into memory filling in prog. Return 0 on success. < 0 on error. */
 int cbfs_prog_stage_load(struct prog *prog);
 
+/*
+ * Data structure that represents "a" CBFS boot device, with optional metadata
+ * cache. Generally we only have one of these, or two (RO and RW) when
+ * CONFIG(VBOOT) is set. The region device stored here must always be a
+ * subregion of boot_device_ro().
+ */
 struct cbfs_boot_device {
 	struct region_device rdev;
 	void *mcache;