Rework `make update-rst`.

Always update the README.rst files when running the command.

Signed-off-by: Tim 'mithro' Ansell <tansell@google.com>

Makefile

diff --git a/Makefile b/Makefile
index fb071b6..e187ffd 100644
--- a/Makefile
+++ b/Makefile
@@ -48,8 +48,10 @@
 			-e's@.. warning::@*Warning*@g' \
 		> $@
 
-update-rst: $(RST_OUT)
+update-rst:
 	@echo Found $(RST_SRC) source files.
+	@touch $(RST_SRC)
+	make $(RST_OUT)
 
 
 COPYRIGHT_HOLDER := SkyWater PDK Authors