Externalization of compilation flags

[jenia81]

Currently, compilation flags can't be configured from mbed-cli.
We want to use compilation flags like:
-Werror, -Wextra,
-Wpointer-arith, -Wcast-align, -Wwrite-strings, -Wswitch-default, -Winit-self, -Wfatal-errors, -Wunreachable-code, -Wlogical-op, -Wmissing-field-initializers, -Wundef, -Wno-unknown-pragmas

[geky]

I believe this is a duplicate of #2001

cc @theotherjimmy, @screamerbg

[ciarmcom]

ARM Internal Ref: IOTMORF-126

[sg-]

This has been implemented with toolchain profiles