ds18b20: Add a C implementation and inprove documentation

This commit adds a C implementation for the DS18B20.  The C++
implementation was untouched (ie: it does not wrap the C
implementation).  This can be done in the future if desired.

In addition, add an ascii-schematic to both the .h and .hpp files to
better illustrate how to wire up the DS 1-wire interface for the UART.

Signed-off-by: Jon Trulson <jtrulson@ics.com>

src/ds18b20/CMakeLists.txt

@@ -1,5 +1,9 @@
-set (libname "ds18b20")
-set (libdescription "upm DS18B20 1-wire Temperature sensor")
-set (module_src ${libname}.cxx)
-set (module_hpp ${libname}.hpp)
-upm_module_init()
+upm_mixed_module_init (NAME ds18b20
+    DESCRIPTION "DS18B20 1-wire Temperature sensor"
+    C_HDR ds18b20.h
+    C_SRC ds18b20.c
+    CPP_HDR ds18b20.hpp
+    CPP_SRC ds18b20.cxx
+#    FTI_SRC ds18b20_fti.c
+#    CPP_WRAPS_C
+    REQUIRES upmc-utilities mraa)