Fix clean build due to change in libopencm3

Commit 48d6693 pulled in an updated libopencm3
for undocumented reasons. In principle this is a good
idea as it fixes a nasty stack initialization bug fixed by
@uhi22 as discussed in
uhi22/ccs32clara#17.
Unfortunately the fix breaks the libopencm3 build for
all platforms except STM32F1.

The fix is to limit the build of libopencm3 to just the
STM32F1 platform in the same way as the CCS32clara
firmware.

Tests:
 - Carry out a full clean build locally
 - Check GitHub CI build after push before raising a PR

Makefile

@@ -143,7 +143,7 @@ ifneq ($(shell test -s libopencm3/lib/libopencm3_stm32f1.a && echo -n yes),yes)
 	@printf "  GIT SUBMODULE\n"
 	$(Q)git submodule update --init
 	@printf "  MAKE libopencm3\n"
-	$(Q)${MAKE} -C libopencm3
+	$(Q)${MAKE} -C libopencm3 TARGETS=stm32/f1
 endif
 
 Test: