Add Gumstix AeroCore device

Based on the work of Andrew Smith [1], add board configuration and device
drivers to support the Gumstix AeroCore (previously Aerodroid) board [2]. The
AeroCore is an autopilot board based on a STM32F427 similar to the FMUv2.

[1] https://github.com/smithandrewc/Firmware
[2] https://store.gumstix.com/index.php/products/585/

Signed-off-by: Ash Charles <ashcharles@gmail.com>

src/systemcmds/mtd/mtd.c

@@ -160,7 +160,11 @@ static void
 ramtron_attach(void)
 {
 	/* find the right spi */
+#ifdef CONFIG_ARCH_BOARD_AEROCORE
+	struct spi_dev_s *spi = up_spiinitialize(4);
+#else
 	struct spi_dev_s *spi = up_spiinitialize(2);
+#endif
 	/* this resets the spi bus, set correct bus speed again */
 	SPI_SETFREQUENCY(spi, 10 * 1000 * 1000);
 	SPI_SETBITS(spi, 8);