add autogen/bootstrap

* This makes it more convenient to invoke autotools. Also MinGW users
  may need to run one of these scripts to be able to compile.
enable silent rules when building

* Note: ONLY configure.ac and stage2/Makefile.am were manually modified.
  The other files were generated by autotools.
fix assembler warnings

* dosstart.S:1356: Warning: using `%ax' instead of `%eax' due to `w' suffix
* grldrstart.S:1277: Warning: using `%ax' instead of `%eax' due to `w' suffix
fix 2 compiler warnings

* char_io.c:283:7: warning: pointer targets in assignment differ in signedness
* fsys_initrd.c:56:13: warning: initialization makes pointer from integer without a cast