external-toolchain: add support for Linaro 2012.01

Linaro has just released a new pre-built toolchain, available as a
tarball, which is a pure toolchain (only the C library is
included). This makes this new Linaro 2012.01 toolchain usable in
Buildroot, so let's integrate the support for it.

In addition to simply adding the new external toolchain at the usual
locations, this patch allows need to adapt a few things to support
Linaro toolchains. Most toolchains store their libraries in the "lib/"
or "usr/lib" directories relative to the toolchain. Buildroot
toolchains on the other hand, store the libraries in the
"usr/<target-name>/lib" directory. And the Linaro toolchain has
choosen to use the "lib/<target-name>" directory. Therefore, this
patch adjust:

 * The logic to search a particular library when that library needs to
   be copied to the target directory

 * The logic to deduce the sysroot directory from the libc.a file
   location in the toolchain: removing "(usr/?)lib(64?)" is no longer
   sufficient, we need to take into account the "lib/<target-name>/"
   case.

Since the Linaro toolchain generates code for Cortex-A processors
only, the selection of this toolchain is limited to Cortex-A8 and
Cortex-A9.

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