Init of rtenv
Rename read() to sys_read() and write() to sys_write()

Reason: Avoid duplicate symbol with semihost
Eliminate removing syscall.s while make clean.

The file does not exist since Sep 21 2013.
Add .gitigore
Rename asm to __asm__ for C99 syntax
Fix compilation warnings
First version of semihost for review. Can not build.
Strip trailing spaces
Implement semihost for rtenv

Known bug: Write extra data in host_write()