distro: use imply to enable DISTRO_DEFAULTS as SoC default

The default of DISTRO_DEFAULTS is messy.  Using the 'imply' keyword
is equivalent and cleaner.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/doc/README.distro b/doc/README.distro
index 2af5590..522deb3 100644
--- a/doc/README.distro
+++ b/doc/README.distro
@@ -165,8 +165,7 @@
 In your board's defconfig, enable the DISTRO_DEFAULTS option by adding
 a line with "CONFIG_DISTRO_DEFAULTS=y". If you want to enable this
 from Kconfig itself, for e.g. all boards using a specific SoC then
-add a "default y if ARCH_FOO" to the DISTRO_DEFAULTS section of
-the Kconfig file in the root of the u-boot sources.
+add a "imply DISTRO_DEFAULTS" to your SoC CONFIG option.
 
 In your board configuration file, include the following: