refactor(console): move putchar() to console driver

Moving putchar() out of libc and adding a weak dummy
implementation in libc.

This is to remove libc's dependencies to the platform
driver.

Signed-off-by: Claus Pedersen <claustbp@google.com>
Change-Id: Ib7fefaec0babb783def614ea23521f482fa4a28a