aii-pxelinux Grub2 support improvements/cleanups

[jouvin]

This issue is a list of improvements/cleanups to be done in aii-pxelinux after #225:

• Turn every function in to a method and use $selffor reporting rather than $this_app. Some adjustments needed to unit tests.
• Use CAF::Path instead of unlink()
• Use @PXE_VARIANTS instead of its contents for loops over variants, to make addition of new variants easier.
• Grub2: implement advanced features like bonding support, append line. Review existing pxelinux_xxx unit tests and ensure they work for Grub2 if appropriate.
• Add a unit test for _link_filepath(), in particular for the specific handling of RESCUE
• Replace the use of -l to test symlink existence by CAF::Path once CAF::Path: add methods to manage links CAF#225 is merged
• Remove value_exists once added to CAF::Application (see Application.pm: no possibility to test if an option exists CAF#210)
• Define Grub2 prefix (set prefix=) in the configuration file rather than relying on the boot loader image being rebuilt with the appropriate prefix. The value to use can probably be computed from nbpdir_grub2 and grub2_efi_kernel_root configuration options.
• Status(): implement a consolidated status, checking the consistency (definition, date) between variants
• hepix_filename(): return undef in case of an error and check the return value before using it.
• remove _exec_userhooks() after aii-ks: ksuserhooks() should handle NoAction flag #256 is fixed
• pxelink.t: improve coverage by testing cases producing errors like missing files for rescue, livecmd or firmware
• Review TODO lines in the code