busybox: pass EXTRA_LDFLAGS for external toolchains

Closes #1321

[Peter: Rework patch to make it apply to git]
Signed-off-by: Grant Edwards <grant.b.edwards@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>