Merge pull request #189 from bright-pan/master

Fix hardfault bug for gcc port
Fix STM32_SRAM_SIZE for stm32f0xx and enable heap management
    for stm32f051x8, STM32_SRAM_SIZE is 8k.
    for stm32f051x4, STM32_SRAM_SIZE is 4k.
Merge remote-tracking branch 'upstream/master'