lm4f: Add USB to serial (CDCACM) example

Implements a USB-to-serial adapter, compliant to the CDCACM subclass.
UART1 is used for the TX/RX lines. Although UART1 also has lines for flow
control, they are not used. Instead, the control lines are implemented in
software via GPIOA.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>

examples/lm4f/stellaris-ek-lm4f120xl/usb_to_serial_cdcacm/README

@@ -0,0 +1,42 @@
+------------------------------------------------------------------------------
+README
+------------------------------------------------------------------------------
+
+This example demonstrates the following:
+ * Using the USB controller
+ * Setting up a simple usb to serial converter (CDCACM)
+
+File list:
+ * usb_cdcacm.c - implementation of the CDCACM subclass
+ * uart.c - implementation of UART peripheral
+ * usb_to_serial_cdcacm.c - glue logic between UART and CDCACM device
+ * usb_to_serial_cdcacm.h - common definitions
+
+
+Implements a USB-to-serial adapter, compliant to the CDCACM subclass. UART1 is
+used for the TX/RX lines. Although UART1 also has lines for flow control, they
+are not used. Instead, the control lines are implemented in software via GPIOA.
+
+The following pinout is used:
+	Tx  <-  PB1
+	Rx   -> PB0
+	DCD  -> PA2
+	DSR  -> PA3
+	RI   -> PA4
+	CTS  -> PA5 (UNUSED)
+	DTR <-  PA6
+	RTS <-  PA7
+
+Note that the CTS pin is unused. The CDCACM specification does not define a way
+to control this pin, nor does it define a way to switch between flow control
+mechanisms.
+
+The glue logic in usb_to_serial_cdcacm.c receives requests from both the UART
+and CDCACM interface, and forward them to their destination, while also
+controlling the LEDs
+
+The green LED is lit as long as either DTR or RTS are high.
+The red LED is lit while the UART is sending data.
+The blue LED is lit while data is read from the UART.
+The red and blue LEDs will only be lit for very short periods of time, thus they
+may be difficult to notice.