toolchain-external: fix the SYSROOT_DIR mangling logic

In a1d94aaa ('toolchain-external: add support for musl C
library'), we made the following change to the SYSROOT_DIR mangling
logic:

-       SYSROOT_DIR=`echo $${LIBC_A_LOCATION} | sed -r -e 's:usr/lib(32|64)?/(.*/)?libc\.a::'` ; \
+       SYSROOT_DIR=`echo $${LIBC_A_LOCATION} | sed -r -e 's:(usr/)?lib(32|64)?/(.*/)?libc\.a::'` ; \

This was needed to accomodate for musl based toolchains that don't
have libc.a in usr/lib/..., but directory in lib/... Basically, the
change makes the usr/ at the beginning optional.

However, with the very permissive (.*) matching in the middle of the
path, the change above had an unexpected consequence: any path contain
'/lib' would be truncated before this lib. As an example, Peter
reported that his builds, running from /var/lib/buildbot/ were no
longer working because the SYSROOT_DIR was decided to be /var instead
of something like
/var/lib/buildbot/buildroot/output/host/opt/ext-toolchain/arm-linux-gnueabihf/libc/.

So, this commit changes (again!) this regexp by changing (.*) to
([^/]*), the idea being that it will match only *one* path
component. Note that this intermediate (.*) directory was added in
e6e60bec ('external-toolchain: add support for Linaro 2012.01') to
accomodate for Linaro toolchains that have a subdirectory in their
sysroot named after the target tuple:

$ ./output/host/opt/ext-toolchain/bin/arm-linux-gnueabihf-gcc -print-file-name=libc.a
/home/thomas/projets/buildroot/output/host/opt/ext-toolchain/bin/../arm-linux-gnueabihf/libc/usr/lib/arm-linux-gnueabihf/libc.a

In addition to this, this commit also makes sure that the change
making usr/ optional is properly reported on all the instances of this
regular expression.

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