package/musl: fix C library installation directory

Set the installation path for the libraries to /lib instead of /usr/lib.

This fixes an issue when building a toolchain with the musl library by
the internal toolchain backend of Buildroot in the first step and import this
toolchain later as a custom external toolchain in a second step. For this use
case check-musl in toolchain/helpers.mk failed because it did not find the
libc or libm in sysroot/lib.

This patch superseeds: [PATCH 1/1] toolchain/helpers.mk: fix check-musl
http://patchwork.ozlabs.org/patch/417587/



Signed-off-by: Jörg Krause <jkrause@posteo.de>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>