nios2: Make NIOS2_CMDLINE_IGNORE_DTB depend on CMDLINE_BOOL

If NIOS2_CMDLINE_IGNORE_DTB is selected but CMDLINE_BOOL isn't and thus
CONFIG_CMDLINE is not defined, the build fails with the following error:

arch/nios2/kernel/setup.c: In function 'nios2_boot_init':
arch/nios2/kernel/setup.c:131:30: error: 'CONFIG_CMDLINE' undeclared
(first use in this function)
   strncpy(boot_command_line, CONFIG_CMDLINE, COMMAND_LINE_SIZE);

This can be reproduced using randconfig with KCONFIG_SEED=0xE5B8749E, i.e.
'make KCONFIG_SEED=0xE5B8749E ARCH=nios2 randconfig'.

Fix this by letting NIOS2_CMDLINE_IGNORE_DTB depend on CMDLINE_BOOL, so we
actually have a default kernel command string to fall back to.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Ley Foon Tan <lftan@altera.com>