rules: provide better feedback on missing libraries.

If the library files are missing (haven't been built) it's better to get a
failure message saying that, rather than a failed dependency with no rules to
generated it.

Before: (I'd manually removed the f4 library)
```
$ make clean all V=1
Using ../../../../../libopencm3/ path to library
rm -f *.o *.d *.elf *.bin *.hex *.srec *.list *.map
make: *** No rule to make target `adc-dac-printf.elf', needed by `elf'.  Stop.
```

After:
```
$ make clean all V=1
Using ../../../../../libopencm3/ path to library
rm -f *.o *.d *.elf *.bin *.hex *.srec *.list *.map
arm-none-eabi-gcc -Os -g -Wextra -Wshadow -Wimplicit-function-declaration
-Wredundant-decls -Wmissing-prototypes -Wstrict-prototypes -fno-common
-ffunction-sections -fdata-sections -MD -Wall -Wundef
-I../../../../../libopencm3//include -DSTM32F4 -mthumb -mcpu=cortex-m4
-mfloat-abi=hard -mfpu=fpv4-sp-d16 -o adc-dac-printf.o -c adc-dac-printf.c
arm-none-eabi-gcc --static -nostartfiles -L../../../../../libopencm3//lib
-T../stm32f4-discovery.ld -Wl,-Map=adc-dac-printf.map -Wl,--gc-sections -mthumb
-mcpu=cortex-m4 -mfloat-abi=hard -mfpu=fpv4-sp-d16 adc-dac-printf.o
-lopencm3_stm32f4 -Wl,--start-group -lc -lgcc -lnosys -Wl,--end-group -o
adc-dac-printf.elf
/home/karlp/tools/gcc-arm-none-eabi-4_9-2015q2/bin/../lib/gcc/arm-none-eabi/4.9.3/../../../../arm-none-eabi/bin/ld:
cannot find -lopencm3_stm32f4
collect2: error: ld returned 1 exit status
make: *** [adc-dac-printf.elf] Error 1
```

I strongly feel that the latter is far more informative.

examples/Makefile.rules

@@ -158,7 +158,7 @@ $(LDSCRIPT):
 	@#printf "  OBJDUMP $(*).list\n"
 	$(Q)$(OBJDUMP) -S $(*).elf > $(*).list
 
-%.elf %.map: $(OBJS) $(LDSCRIPT) $(LIB_DIR)/lib$(LIBNAME).a
+%.elf %.map: $(OBJS) $(LDSCRIPT)
 	@#printf "  LD      $(*).elf\n"
 	$(Q)$(LD) $(LDFLAGS) $(ARCH_FLAGS) $(OBJS) $(LDLIBS) -o $(*).elf