[cpu][sam926] Using MMU to map vector table before init INT controller.

1. Cleanup unused code in startup.c.
2. Init MMU before init INT contoroller for mapping vector table to visual address 0x0
3. Remove unused section in link scripts.

1. Combine code for IAR and GCC in file mmu.c and cpuport.c
2. Remove remap code in start_xxx.S. User should config MMU to map vector table to visual address 0x0

[bsp][sam9260] Add GDB start scripts. 

[bsp][stm32f0x] Modify build config to reduce code size for Travis CI

x86: fix compile error in c++

[bsp][stm32f03x] Modify build config to reduce code size for Travis CI

[cpu] Split ARM926 cpu code from SAM9260 BSP

2. [bsp][sam9260] Remove unused ld file
3. [bsp][sam9260] Add J-Link debug scripts

1. [bsp][sam9260] Fix the bug that auto reset after boot 20s. Disable watchdog in rt_lovel_level_init function.
2. [bsp][sam9260] Modify SCONS scripts to support IAR tool chain.
3. [bsp][sam9260] Move link strips in to folder link_scripts.
4. [libcpu][arm926] Add copy right to source file and format code.

Fix a regression on 62a0172d.

The old `--keep` parameter for Keil is wrong. RTFM of Keil and get it
right.

Fix keil keep symbol

Fix a regression on 62a0172d.

The old `--keep` parameter for Keil is wrong. RTFM of Keil and get it
right.

[DFS] fix the NFS link issue

[tools] Add package.json as building script.

memheap: check the USED bit in rt_memheap_free

This will ease the debugging of double-free bug.

修正rtt-root位于其他目录时计算目录错误的bug

Use svg instead of png to get better image quality

[DeviceDriver] stm32: Remove explicity clear of RXNE flag

According to STM32 Manual, the USART RXNE flag will be clear automatically after
read to the USART_DR register[1], so the call to USART_ClearITPendingBit is
unnecessary.

[1]: See RM0090 Reference Manual p.992, Bit 5