fix(console): correct scopes for console symbols

fix(console): correct scopes for console symbols

console_state is only used in multi_console.c, it is then declared as
static. console_list is used by several files, declare it as extern.
This corrects the 2 sparse warnings:
drivers/console/multi_console.c:13:11: warning: symbol 'console_list'
 was not declared. Should it be static?
drivers/console/multi_console.c:14:9: warning: symbol 'console_state'
 was not declared. Should it be static?

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: Id1891595906c258e8cb8aa325226f0a43723ca0e