expo: Add basic implementation

An expo is a way of presenting and collecting information from the
user. It consists of a collection of 'scenes' of which only one is
presented at a time. An expo is typically used to show a boot menu
and allow settings to be changed.

One created, the same expo can be automatically presented in graphical
form using a vidconsole, or in text form on a serial console.

Add an initial implementation of the expo itself. Supports for scenes
and objects is provided later.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/tools-only_defconfig b/configs/tools-only_defconfig
index de99f38..2197063 100644
--- a/configs/tools-only_defconfig
+++ b/configs/tools-only_defconfig
@@ -7,6 +7,8 @@
 CONFIG_FIT=y
 CONFIG_TIMESTAMP=y
 CONFIG_FIT_SIGNATURE=y
+# CONFIG_BOOTSTD_FULL is not set
+# CONFIG_BOOTMETH_VBE is not set
 CONFIG_USE_BOOTCOMMAND=y
 CONFIG_BOOTCOMMAND="run distro_bootcmd"
 # CONFIG_CMD_BOOTD is not set