toolchain/external: commonalise comments about Linaro toolchains

Those two comments:
  - are exactly the same
  - have the same dependencies (except for arm/armeb)

So, make it a common comment. It will be useful to have that comment
when we introduce new Linaro toolchain versions.

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