arm: socfpga: Switch CONFIG_HOSTNAME to CONFIG_SYS_BOARD

We already have the CONFIG_SYS_BOARD variable, which defines the name
of the board. The value in CONFIG_HOSTNAME is exactly the same and is
thus just a duplicity, so switch it to reuse CONFIG_SYS_BOARD .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Chin Liang See <clsee@altera.com>
Cc: Dinh Nguyen <dinguyen@opensource.altera.com>