efi_loader: Allow to compile helloworld.efi w/o bundling it

Today we can compile a self-contained hello world efi test binary that
allows us to quickly verify whether the EFI loader framwork works.

We can use that binary outside of the self-contained test case though,
by providing it to a to-be-tested system via tftp.

This patch separates compilation of the helloworld.efi file from
including it in the u-boot binary for "bootefi hello". It also modifies
the efi_loader test case to enable travis to pick up the compiled file.
Because we're now no longer bloating the resulting u-boot binary, we
can enable compilation always, giving us good travis test coverage.

Signed-off-by: Alexander Graf <agraf@suse.de>
Reviewed-by: Tom Rini <trini@konsulko.com>