doc: Mention fastboot dependency on CMDLINE

The fastboot 'boot' command only supports running a U-Boot command if
CONFIG_CMDLINE is enabled. Mention this in the documentation.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Sam Protsenko <semen.protsenko@linaro.org>
diff --git a/doc/android/fastboot.rst b/doc/android/fastboot.rst
index 1ad8a89..933a652 100644
--- a/doc/android/fastboot.rst
+++ b/doc/android/fastboot.rst
@@ -127,6 +127,7 @@
 
 When executing the fastboot ``boot`` command, if ``fastboot_bootcmd`` is set
 then that will be executed in place of ``bootm <CONFIG_FASTBOOT_BUF_ADDR>``.
+This is supported if CONFIG_CMDLINE is enabled, which it normally is.
 
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