Add default heap size( 1 KB) for Nucleo F401RE, DISCOVERY 407

- Add default heap size definition in startup asm file
hmm... I thought that startup file and linker script (scatter loading description file?) don't seem to have any definition for memory map. so I have copied some from another stm32f4 family startup code. does it need to have those kind of definition in linker script or assembler file?