extra-build-config.md: added a white line

There was in the rendered MarkDown no new line between the bullet list and the next line.

Signed-off-by: Jan Vermaete <jan.vermaete@gmail.com>

docs/extra-build-config.md

@@ -178,6 +178,7 @@ the following boards:
 * Raspberry Pi 3 64-bit
 * Raspberry Pi 4 32-bit
 * Raspberry Pi 4 64-bit
+
 It means that, for those boards, `RPI_USE_U_BOOT = "1"` is not compatible with
 `ENABLE_UART = "0"`.