As in the main library, instead of constructing and making our own CFLAGS
CXXFLAGS and LDFLAGS variables, use TGT_ for ones we construct, and include any
user provided variables afterwards.
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.
This example is just using buffers and built in alpha overlay
functionality to animate a dmond floating on a checker board. After
initializing of the frame buffers only 7 registers are being modified to
implement the animation.
User can specify which binaries he needs on the commandline.
'make hex' will build examples with generating hex file
'make hex bin' will build hex and bin output
'make images' will build all images as in previous versions of buildsystem
'make all' or 'make' will build only elf (+ map) files
This will have huge impact on build speed.
