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>

package/binutils/Config.in.host

@@ -2,6 +2,7 @@ comment "Binutils Options"
 
 choice
 	prompt "Binutils Version"
+	default BR2_BINUTILS_VERSION_2_28_X if BR2_ARM_CPU_ARMV7M
 	default BR2_BINUTILS_VERSION_2_29_X if !BR2_arc
 	default BR2_BINUTILS_VERSION_ARC if BR2_arc
 	help