Instead of matching whatever ABI we might be compiled against like
previously, define the kernel ABI we need as a prerequisite
(EVL_KABI_PREREQ), checking for sanity at build time and runtime.

This prerequisite is matched against the range of ABI revisions the
kernel supports (from EVL_ABI_BASE to EVL_ABI_CURRENT). In the
simplest case, the kernel implements a single ABI with no backward
compatibility mechanism (EVL_ABI_BASE == EVL_ABI_CURRENT).

This addresses two issues:

- the fact that libevl might build against a given set of uapi/ files
  does not actually mean that the corresponding kernel ABI found there
  is fully compatible with what libevl expects. Specifying a
  compatible ABI prereq explicitly addresses this problem.

- we can obtain services from EVL cores supporting multiple ABI
  revisions (i.e. providing backward compat feat).

Signed-off-by: Philippe Gerum <rpm@xenomai.org>