toolchain-external: do not pass TOOLCHAIN_EXTERNAL_CFLAGS for kernel headers check

When getting the sysroot used for the kernel headers version check,
passing TOOLCHAIN_EXTERNAL_CFLAGS causes a problem when used with
multilib toolchains, where only the main sysroot has the header files,
and the other sysroots only have the libraries.

Since the kernel headers version used is normally the same for all
sysroots, this commit solves this problem by removing the
TOOLCHAIN_EXTERNAL_CFLAGS argument when calling
toolchain_find_sysroot, so that it returns the main sysroot, in which
<linux/version.h> can be found for the kernel headers version check.

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>