bootstd: Allow bootmeths to be marked as global

The current way of handling things like EFI bootmgr is a bit odd, since
that bootmeth handles selection of the bootdev itself. VBE needs to work
the same way, so we should support it properly.

Add a flag that indicates that the bootmeth is global, rather than being
invoked on each bootdev. Provide a helper to read a bootflow from the
bootmeth.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/efi_loader/Kconfig b/lib/efi_loader/Kconfig
index e3f2402..5cfff8c 100644
--- a/lib/efi_loader/Kconfig
+++ b/lib/efi_loader/Kconfig
@@ -37,6 +37,7 @@
 config CMD_BOOTEFI_BOOTMGR
 	bool "UEFI Boot Manager"
 	default y
+	select BOOTMETH_GLOBAL if BOOTSTD
 	help
 	  Select this option if you want to select the UEFI binary to be booted
 	  via UEFI variables Boot####, BootOrder, and BootNext. This enables the