build: simplify subdirectory detection for EAL

build: simplify subdirectory detection for EAL

Within EAL we had a series of if statements for selecting the EAL directory
to use. Now that the directory names match those of the OS's they are for
we can instead just use a generated subdirectory name, shortening the code.
To avoid strange errors, we still need to check for unsupported OS's, but
do this check up-front in the config meson.build file.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
Reviewed-by: David Marchand <david.marchand@redhat.com>
Acked-by: Luca Boccassi <bluca@debian.org>