console.c: fix problem with splashimage

If a board uses cfb_console driver and splash image
and also defines CONFIG_SILENT_CONSOLE, the user is
locked out even if "silent" is not set. It is not
possible to get any output, neither on vga console
device nor on serial console after redirecting the
output to the serial console, since the GD_FLG_SILENT
flag remains set.

Fix the problem by redirecting the output from frame
buffer to serial console if splashimage is used.
Only suppress the output if "silent" environment
variable was set and don't set the GD_FLG_SILENT
flag arbitrarily.

Signed-off-by: Anatolij Gustschin <agust@denx.de>