device: don't set a default cross compile prefix

device: don't set a default cross compile prefix

qmake -set can be used instead to set a the default cross compile.
device_config.prf already contains code to read this default.

Remove per-spec CROSS_COMPILE checks

Introduce deviceSanityCheckCompiler() usage where appropriate

Done-with: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>
Change-Id: I07c75c9e933dc1174a1bf8bf523b6b4a6b427408
Reviewed-by: Romain Pokrzywka <romain.pokrzywka@kdab.com>
Reviewed-by: Girish Ramakrishnan <girish.1.ramakrishnan@nokia.com>