Fix the undefined symbol NRF_SRAM_END
Configure SRAM part for Heap

Add interrupt enter and leave

iir = LPC_Uart->LSR;	//oe pe fe oe read for clear interrupt

Export the end address of ram, to avoid hard-code heap size in application

Use *.icf ram symbal, to avoid hardcode.

fixed bug: 'scons --target=iar -s' output iar project missing startup code, and link file

修复scons生成IAR工程中没有startup code和链接文件，导致工程中断异常和Finsh无法工作

1. Fixed the bug that MPU init function was called 2 times.

1. Add IAR project template and link scripts. Add missing CXX configuration for IAR tool chain in rtconfg.py.
2. Modify all link scripts, Using ITCM interface.(Changing flash start address from 0x0800_0000 to 0x0020_0000).

1. Using RT_USING_COMPONENTS_INIT and RT_USING_USER_MAIN
2. Cleanup code

code cleanup.