package/binutils: default to 2.29 for Cortex-M targets

binutils 2.29 changed the implementation of adr pseudo instruction
it breaks linux kernel and impacts Cortex-M targets (eg. stm32)

[Peter: simplify Config.in logic, adjust message to make it clear this is
	just a default]
Signed-off-by: Laurent GONZALEZ <br22@gezedo.com>
Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>