MIPS: Always define builtin_cmdline

Commit 7784cac6 ("MIPS: cmdline: Clean up boot_command_line
initialization") made use of builtin_cmdline conditional upon plain C if
statements rather than preprocessor #ifdef's. This caused build failures
for configurations with CONFIG_CMDLINE_BOOL=n where builtin_cmdline
wasn't defined, for example:

   arch/mips/kernel/setup.c: In function 'bootcmdline_init':
>> arch/mips/kernel/setup.c:582:30: error: 'builtin_cmdline' undeclared
    (first use in this function); did you mean 'builtin_driver'?
      strlcpy(boot_command_line, builtin_cmdline, COMMAND_LINE_SIZE);
                                 ^~~~~~~~~~~~~~~
                                 builtin_driver
   arch/mips/kernel/setup.c:582:30: note: each undeclared identifier is
    reported only once for each function it appears in

Fix this by defining builtin_cmdline as an empty string in the affected
configurations. All of the paths that use it should be optimized out
anyway so the data itself gets optimized away too.

Signed-off-by: Paul Burton <paul.burton@mips.com>
Fixes: 7784cac6 ("MIPS: cmdline: Clean up boot_command_line initialization")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Nathan Chancellor <natechancellor@gmail.com>
Cc: linux-mips@vger.kernel.org