.gitignore: ingore files generated by Kbuild

Ignore generated files by Kbuild such as .*.cmd, *.order, etc.

Besides above,
 - Ignore *.s files
   We do not need to ignore with file name, asm-offsets.s
 - Do not ignore *.rej (for quilt)
 - Ignore backup files, \#*#

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>