configure: rationalise error messages with meaningful texts

Currently, error messages just state the obvious: a required
component is required. While in some cases, there is not much
else to say, in some other cases we could tell much more.

For example, a missing libstdc++ means no static toolchain.

Also, format error and warning messages in a similar fashion,
with a {error,warning} intro, followed by the message specified
by the caller.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # Dummy config file to not use a C library *at all*

 ## depends on BARE_METAL

 ##

 ## help Do not use a C library.

 ## help

 ## help This is usefull if your project is self-contained, does not rely on

 ## help an external C library, and provides all the necessary bits.

 ## help

 ## help Most probably usefull to bootloaders, as they generally don't depend

 ## help on the C library.

 ## help

 ## help If unsure: do *not* choose that, and use another option in the choice.