util: support "y" and "n" in env_var_as_boolean()

Suggested-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
10 jobs for !1158 with debug-y in 5 seconds (queued for 1 second)
detached
Status Job ID Name Coverage
  Containers Build
canceled #377896
debian

00:00:05

 
  Build+Test
skipped #377898
meson-clang
skipped #377902
meson-clover
skipped #377901
meson-main
skipped #377897
meson-swr-glvnd
skipped #377903
meson-vulkan
skipped #377905
scons-llvm
skipped #377904
scons-nollvm
skipped #377899
scons-swr
skipped #377900
scons-win64