extra-build-config.md: add RPi 0W to UART section

This makes it clear in the docs that ENABLE_UART can be used both for the RPi 0W or 3.

(From meta-raspberrypi rev: 34dab90dfc1b518cd9235e2fbb8e0e9ee0ecd263)

Change-Id: I439a21675ac5c004fe00ec4cb2bc25f59f234121
Signed-off-by: Bernardo A. Rodrigues <bernardaraujor@gmail.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-raspberrypi/docs/extra-build-config.md b/meta-raspberrypi/docs/extra-build-config.md
index b44dece..426423a 100644
--- a/meta-raspberrypi/docs/extra-build-config.md
+++ b/meta-raspberrypi/docs/extra-build-config.md
@@ -219,7 +219,7 @@
 operations - 60fps h264 decode, high quality deinterlace - which aren't
 performed on the ARM may be affected, and we wouldn't want to do that to users
 who don't want to use the serial port. Users who want serial console support on
-RaspberryPi3 will have to explicitly set in local.conf:
+RaspberryPi 0 Wifi or 3 will have to explicitly set in local.conf:
 
     ENABLE_UART = "1"