compiler: gcc: Add warning option to spot poblematic switches

Option -Wimplicit-fallthrough as the name suggests raises a warning when
there is an implicit fallthrough in a switch clause. This warning does
not happen when the last statement is a return or a call to a function
declared with noreturn attribute.

This flag helps to enforce two MISRA-C rules (not completely), 16.1 and
16.3

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>