Clarify toolchains and install steps

Including removing the extremely unhelpful DETECT_TOOLCHAIN option.
sync: use matching #ifdefs in source as in header

Fixes github issue #298
stm32f1: Fix RCC CAN defines

Thanks to Марко Краљевић <krasnaya.zvezda@gmail.com>
[f3] add USART_SR_x defines for common status flags

enables the use of usart_get_flag(USARTx, USART_SR_x) on F3 just like on F124

closes #283
libopencmsis: Fill in missing CMSIS interrupt links

And correct a minor typo in the generated code.
stm32/flash: Make comments match reality

Copy/paste commentary with wrong sizes.
stm32/dac: Follow ref-man guidelines on GPIO config

GPIO pins should be configured for Analog IN on virtually all families
to reduce parasitic consumption.

Reported-by: Roger Wolff <R.E.Wolff@BitWizard.nl>
stm32/f0: DMA base address compatibility tweak
usb/stm32 added NOVBUSSENS bit definitions
usb/stm32: Add top level commentary for scope

Remove some unncessary commentary and fixed bit fields introduced earlier via
insufficient review.
[stm32] implement i2c_reset using rcc_periph_reset_pulse

this also adds support for I2C3 in i2c_reset
[stm32] fix i2c_reset

rcc_periph_reset_pulse needs RST_I2Cx instead of RCC_I2Cx as argument
see also #329
[stm32] fix dox for rcc_periph_x

closes #329