package/kexec-lite: Add a package for the kexec-lite tools

Kexec-lite is a tiny impementation of kexec for devicetree-based
platforms.

[Thomas:
- Add !BR2_PREFER_STATIC_LIB dependency, inherited from the dtc
  package
- Fix license to be GPLv2+, and not just GPL.
- Use $(TARGET_CONFIGURE_OPTS) instead of manually passing CC, LD,
  CFLAGS.
- Use a full path as the target of $(INSTALL)]

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>