Add support for 'qmod' (quad-modem) board

You can compile the firmware for this new target using 'make
BOARD=qmod'.

The QMOD differs from simtrace+owhw in terms of low-level
initialization, as it has an external 12MHz clock at XIN, thus bypasses
the main oscillator and needs different PLL configuration.

firmware/include_board/owhw/board.h

@@ -6,6 +6,8 @@
 /** Board definition */
 #define owhw
 
+#define BOARD_MAINOSC 18432000
+
 /* USIM 2 interface (USART) */
 #define PIN_USIM2_CLK		{PIO_PA2, PIOA, ID_PIOA, PIO_PERIPH_B, PIO_DEFAULT}
 #define PIN_USIM2_IO		{PIO_PA6, PIOA, ID_PIOA, PIO_PERIPH_A, PIO_DEFAULT}