package/binutils: extend the 2.28 default to thumb mode

Commit 17f352ac (package/binutils: default to 2.29 for Cortex-M targets)
made the default version 2.28 (and not 2.29!) when the target is an
arm-v7m CPU.

However, the real trigger is compilation in Thumb mode, not the fact
that the target is v7m.

The fact that it was noticed on a v7m target is because Thumb is the
only mode valid on those CPUs.

Tighten the defaults to 2.28 for Thumb and Thumb2 modes.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Laurent GONZALEZ <br22@gezedo.com>
Cc: Christophe Priouzeau <christophe.priouzeau@st.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>