Merge pull request #70 from zzz0072/master

Documentation: add build system Internals
add support USART1 & USART6 in usart_baud(), because USART1 & USART6 clock source is APB2/PCLK2, others USARTx is APB1/PCLK1

current the kernel setting STM32F4xx CPU Clock/AHB (HCLK) run in 168MHz
PCLK1: 42MHz, PCLK2: 84MHz

reference: Table 2. STM32F4xx register boundary RM0090 Reference manual
(DM00031020.pdf, http://www.st.com/web/en/resource/technical/document/reference_manual/DM00031020.pdf)
add DBGPORT_USE_USART1 options
Should pick up it for STM32F429I discovery board
Merge pull request #69 from KunYi/master

platform: add USART1 configurations for STM32F4x
update USART configurations for STM32F4x
remove all trailing blank lines and all trailing whitespace

Tips from RIOT-OS: https://github.com/RIOT-OS/RIOT/pull/673

Remove all the trailing whitespace:
  find * -type f -exec sed -i -E 's/[[:space:]]*$//' {} \;

Remove the last trailing blank line:
  for file in $(find * -type f -exec sh -c '[ -z "$(sed -n "\$p" $1)" ] &&
  echo "$1"' _ {} \;); do
    head --lines=-1 ${file} | sponge ${file};
  done
Move loader.mk to mk/rules and rmove misleading target.mk file

Fix #65
Merge pull request #71 from georgekang/master

mk: move loader.mk to directory rules and remove misleading target.mk file