bootstd: Add the concept of a bootflow

A bootflow encapsulates the process used to boot an operating system.
It typically has a control file (such as extlinux.conf) and information
about which 'bootdev' it came from.

Add the header file for this first, since it is needed by all other
files.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index f7665fc..d6d4557 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -693,6 +693,11 @@
 S:	Maintained
 F:	tools/binman/
 
+BOOTDEVICE
+M:	Simon Glass <sjg@chromium.org>
+S:	Maintained
+F:	include/bootflow.h
+
 BTRFS
 M:	Marek Behun <marek.behun@nic.cz>
 R:	Qu Wenruo <wqu@suse.com>