fastboot: add UUU command UCmd and ACmd support

add support for the UUU commands ACmd and UCmd.

Enable them through the Kconfig option
CONFIG_FASTBOOT_UUU_SUPPORT

base was commit in NXP kernel
9b149c2a2882: ("MLK-18591-3 android: Add FSL android fastboot support")

and ported it to current mainline. Tested this patch
on imx6ul based board.

Signed-off-by: Heiko Schocher <hs@denx.de>
Acked-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/doc/android/fastboot-protocol.rst b/doc/android/fastboot-protocol.rst
index e723659..e8cbd7f 100644
--- a/doc/android/fastboot-protocol.rst
+++ b/doc/android/fastboot-protocol.rst
@@ -144,6 +144,11 @@
 
   "powerdown"          Power off the device.
 
+  "ucmd"               execute any bootloader command and wait until it
+                       finishs.
+
+  "acmd"               execute any bootloader command, do not wait.
+
 Client Variables
 ----------------
 
diff --git a/doc/android/fastboot.rst b/doc/android/fastboot.rst
index ce513a2..7611f07 100644
--- a/doc/android/fastboot.rst
+++ b/doc/android/fastboot.rst
@@ -19,6 +19,8 @@
 - ``reboot``
 - ``reboot-bootloader``
 - ``set_active`` (only a stub implementation which always succeeds)
+- ``ucmd`` (if enabled)
+- ``acmd`` (if enabled)
 
 The following OEM commands are supported (if enabled):