Add support for xenomai 3.5.7 kernel

- add kernel to debian/configure script
  - 'RTFLAVOR' variable controls RTAI or Xenomai build
- move Module.symvers into /lib/modules/<kver>/ethercat/
  - near the kernel Module.symvers; generic to all RT flavors
- patch to locate Xenomai headers more correctly
  - ./configure --with-xenomai  [ --with-xeno-config=<path> ]
  - locate xeno-script; query it for Xenomai CFLAGS
  - use those CFLAGS for rtdm.c
install ecrt.h and ectty.h in /usr/include/ethercat

Install headers into /usr/include/ethercat rather than /usr/include so
that kbuild doesn't need to add '-I/usr/include' to CFLAGS!

- patch include/Makefile.am
- remove links into kernel
initial commit of debian/.gitignore

.gitignore package build artifacts