Issue #340, add bitmap/hello_world example

This example uses the Bitmap device to draw to an in-memory buffer
and writes the result to a TGA file. This can for example be used to
make screenshots of an application normally running on an Arduino.

sys/bitmap/hello_world/Makefile

@@ -0,0 +1,12 @@
+CFLAGS = -g -Wall -I../../../csrc/.
+
+SRC = $(shell ls ../../../csrc/*.c) $(shell ls ../common/u8x8_d_bitmap.c ) main.c
+
+OBJ = $(SRC:.c=.o)
+
+hello_world: $(OBJ)
+	$(CC) $(CFLAGS) $(LDFLAGS) $(OBJ) -o $@
+
+clean:
+	-rm -f $(OBJ) hello_world *.tga
+

sys/bitmap/hello_world/main.c

@@ -0,0 +1,40 @@
+#include "u8g2.h"
+#include <stdio.h>
+
+/*
+ * This example uses the Bitmap device to draw to an in-memory buffer
+ * and writes the result to a TGA file. This can for example be used to
+ * make screenshots of an application normally running on an Arduino.
+ */
+
+u8g2_t u8g2;
+
+int main(void)
+{
+  u8g2_SetupBitmap(&u8g2, &u8g2_cb_r0, 128, 64);
+  u8x8_InitDisplay(u8g2_GetU8x8(&u8g2));
+  u8x8_SetPowerSave(u8g2_GetU8x8(&u8g2), 0);
+  u8g2_SetFont(&u8g2, u8g2_font_helvB08_tr);
+
+  u8g2_ClearBuffer(&u8g2);
+  // Here, you can call other code to actually draw things, such as
+  // existing application code to generate a screenshot
+  u8g2_DrawStr(&u8g2, 10, 30, "Hello, world!");
+  u8g2_SendBuffer(&u8g2);
+  u8x8_SaveBitmapTGA(u8g2_GetU8x8(&u8g2), "small.tga");
+
+  // This changes size, but also resets other stuff like font
+  u8g2_SetupBitmap(&u8g2, &u8g2_cb_r0, 128, 128);
+  u8g2_SetFont(&u8g2, u8g2_font_helvB08_tr);
+
+  u8g2_ClearBuffer(&u8g2);
+  // Here, you can call other code to actually draw things, such as
+  // existing application code to generate a screenshot
+  u8g2_DrawStr(&u8g2, 10, 30, "Hello, world!");
+  u8g2_DrawStr(&u8g2, 10, 100, "Hello, larger world!");
+  u8g2_SendBuffer(&u8g2);
+  u8x8_SaveBitmapTGA(u8g2_GetU8x8(&u8g2), "large.tga");
+
+  return 0;
+}
+