efi_loader: rename __efi_hello_world_*

In scripts/Makefile.lib we build section including helloworld.efi.
This allows to load the EFI binary with command 'bootefi hello'.

scripts/Makefile.lib contains explicit references to strings
containing helloworld and hello_world. This makes it impossible
to generalize the coding to accomodate additional built in
EFI binaries.

Let us rename the variables __efi_hello_world_* to
__efi_helloworld_*.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Signed-off-by: Alexander Graf <agraf@suse.de>