grub2: pass target NM, OBJCOPY and STRIP

Fixes https://bugs.busybox.net/show_bug.cgi?id=8256



grub2's build system interprets CC, CFLAGS and CPPFLAGS as for the host
and uses TARGET_CC etc. for the target. However, NM, OBJCOPY and STRIP
are used for the target. We currently pass the host-versions of these
tools as part of $(HOST_CONFIGURE_OPTS).

While we're at it, also pass TARGET_LDFLAGS.

This problem had not been noticed up to now because usually we build on
an x86 machine for the x86 architecture, so the binutils are compatible.
However, this is not true on an i386 when building for x86_64.

Cc: Christophe Bricout <christophebricout@yahoo.fr>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>