bootstd: Add an implementation of distro boot

Add a bootmeth driver which handles distro boot from a disk, so we can
boot a bootflow using this commonly used mechanism.

In effect, this provides the same functionality as the 'sysboot' command
and shares the same code. But the interface into it is via a bootmeth.

For now this requires the 'pxe' command be enabled. Future work may tidy
this up so that it can be used without CONFIG_CMDLINE being enabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 5a48b84..d9125fc 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -701,7 +701,8 @@
 F:	boot/bootmeth*.c
 F:	boot/bootstd.c
 F:	cmd/bootdev.c
-F:	include/bootdev*.h
+F:	cmd/bootflow.c
+F:	include/bootdev.h
 F:	include/bootflow.h
 F:	include/bootmeth.h
 F:	include/bootstd.h