toolchain-external: move the sysroot check to helper function

toolchain-external: move the sysroot check to helper function

The sysroot toolchain support check is duplicated at two locations in
the external toolchain infra. So move it inside the
check_unusable_toolchain helper that is called when the toolchain
package is configured (TOOLCHAIN_EXTERNAL_CONFIGURE_CMDS).

The check in TOOLCHAIN_EXTERNAL_INSTALL_SYSROOT_LIBS can be safely
removed since it's already done in check_unusable_toolchain helper.

The check in TOOLCHAIN_EXTERNAL_INSTALL_TARGET_LIBS was removed by
2a87b64f8e8a832a351f8552eb032e4b18431949.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>