toolchain-external: fix Linaro ARM toolchain support

This commit fixes bug #6452 (eglibc from Linaro 2013.07 not copied to
target correctly) by:

 * Copying only the relevant library loader to the target on ARMhf
   (i.e ld-linux-armhf.so and not ld.so*). This is needed since Linaro
   toolchains provide two library loaders, one ARMv7 hf, and one ARMv4
   soft-float.

 * Making sure a $(TARGET_DIR)/lib/arm-linux-gnueabihf/ symbolic link
   to $(TARGET_DIR)/lib/ exists, since the dynamic loader of Linaro
   toolchains expects libraries to be found in
   $(TARGET_DIR)/lib/arm-linux-gnueabihf/.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>