toolchain: set the ssp gcc option in kconfig

toolchain: set the ssp gcc option in kconfig

Currently, we repeat all the SSP level selection deep down to the
toolchain wrapper itself, where we eventually translate it to the
actual SSP option to use. This is a bit redundant.

Additionally, we will want to check that the toolchain actually
supports that option (for those toolchain where it was backported).

So, move the translation into kconfig, and add the qstrip'ed value
to the additional flags passed to the wrapper. Add it before
user-supplied opitons, to keep the previous behaviour (and allow
anyone crazy-enough to override it with BR2_TARGET_OPTIMIZATION).

Signed-off-by: "Yann E. MORIN" <yann.morin@orange.com>
Cc: Matt Weber <matthew.weber@rockwellcollins.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Reviewed-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>