New board: stm32f3_discovery

- board makefile
- Ld script for stm32f3discovery
Now libopencm3 compiles support for the f3, also Makefile.stm32 updated
[stm32f3] uart for F3
[stm32f3discovery] Uart fixed for spektrum arch
[stm32f3discovery] Memory size 256Kb
[stm32f3] i2c arch support added
[stm32f3] gpio for f3
[stm32f3discovery] main cpu freq corrected
[stm32f3] actuators_pwm_arch updated for f3
[stm32f3] spi support added
[subsystems] add imu stm32f3_discovery
[conf] stm32f3_discovery_pwm airframe file
update libopencm3 to get STM32F3 fixes
[boards] some updates for stm32f3_discovery
[stm32f3] i2c_400khz_i2cclk8mhz
[stm32f3] add adc support, untested!
[conf] flash_modes: add stm32f3_discovery board
[stm32] fix spektrum arch for non F3