Use already qstripped BR2_TOOLCHAIN_EXTERNAL_PREFIX everywhere

Use already qstripped BR2_TOOLCHAIN_EXTERNAL_PREFIX everywhere

The BR2_TOOLCHAIN_EXTERNAL_PREFIX variable is already qstripped and
stored in the TOOLCHAIN_EXTERNAL_PREFIX variable in
toolchain-external.mk, so use this variable everywhere.

This will be useful for a later patch that makes the derivation of
TOOLCHAIN_EXTERNAL_PREFIX a little more complex.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Arnout: split off into separate patch]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>