toolchain-external: bump version of Linaro ARM toolchain to 7.2-2017.11

https://releases.linaro.org/components/toolchain/binaries/7.2-2017.11



Tested with qemu_arm_vexpress_defconfig.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>