eglibc: enable support in the Buildroot toolchain backend

Using the newly introduced 'eglibc' package, this commit enables the
option of building a toolchain using the eglibc C library in the
Buildroot toolchain backend.

In details, this commit:

 * Creates a choice to select uClibc or eglibc in the Buildroot
   toolchain backend (in toolchain/toolchain-buildroot/Config.in), and
   removes the fact that the Buildroot toolchain backend forcefully
   enables uClibc (toolchain/Config.in).

 * Creates a BUILDROOT_LIBC variables, which points to the package
   implementing the C library (i.e either 'uclibc' or 'eglibc').

 * Modifies the gcc-final and gcc-intermediate makefiles to use the
   BUILDROOT_LIBC variable instead of hardcoding the use of uclibc.

 * Ensures that TLS support is always enabled when building eglibc.

[Peter: fix commit text to refer to BUILDROOT_LIBC]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>