libpayload: Add boot_device_read() function

This patch adds a new way of implementing flash access for libpayload.
Until now all reads had to be performed using cbfs_media, which is
obsolete. From now on all reads should be performed using
boot_device_read().

This patch also provides a default implementation of boot_device_read()
for x86 architecture.

Change-Id: I1babd2a8414ed9de3ca49903fcb4f036997b5ff3
Signed-off-by: Jakub Czapiga <jacz@semihalf.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/59492
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Julius Werner <jwerner@chromium.org>

payloads/libpayload/include/boot_device.h

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
+
+#ifndef _BOOT_DEVICE_H
+#define _BOOT_DEVICE_H
+
+#include <stddef.h>
+
+/**
+ * This is a boot device access function, which is used by libpayload to read data from
+ * the flash memory (or other boot device). It has to be implemented by payloads that want
+ * to use FMAP or libcbfs.
+ *
+ * @param buf The output buffer to which the data should be written to.
+ * @param offset Absolute offset in bytes of the requested boot device memory area. Not aligned.
+ * @param size Size in bytes of the requested boot device memory area. Not aligned.
+ *
+ * @returns Number of bytes returned to the buffer, or negative value on error. Typically should
+ *          be equal to the `size`, and not aligned forcefully.
+ */
+ssize_t boot_device_read(void *buf, size_t offset, size_t size);
+
+#endif /* _BOOT_DEVICE_H */