includes: add strchra() and strchrnula() impls
includes: add strchra() and strchrnula() impls

Unfortunately GNU-EFI doesn't currently implement ascii versions of
strchr() or strchrnul(), and we kind of need them, so add an
implementation here for now.

Signed-off-by: Peter Jones <2e0a021e603479ff81838c00c3b770daca4e0214@redhat.com>

rhinstaller/shim,rhinstaller/shim,rhinstaller/shim