libmaple/stm32.h: add a FIXME for USART config

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>
systick.h: add systick_attach_callback() prototype

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>
nvic.h: silence some GCC inline warnings

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>
HardwareSPI.cpp: allow for missing SPI peripherals

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>
HardwareTimer.cpp: use STM32_TIMER_MASK

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>
wirish: allow boards to override more init() subroutines

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>
wirish: allow boards to override PLL input clock

Signed-off-by: Marti Bolivar <mbolivar@leaflabs.com>