MIPS: Make builtin_cmdline const & variable length

We have no need for the builtin_cmdline array to be fixed at the length
of COMMAND_LINE_SIZE - we'll only copy out the string it contains up to
its NULL terminator anyway, and cap the size at COMMAND_LINE_SIZE when
copying into or concatenating with boot_command_line.

The string value is also constant, so we can declare it as such to place
it in the .init.rodata section.
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org

arch/mips/kernel/setup.c

@@ -67,7 +67,7 @@ static char __initdata command_line[COMMAND_LINE_SIZE];
 char __initdata arcs_cmdline[COMMAND_LINE_SIZE];
 
 #ifdef CONFIG_CMDLINE_BOOL
-static char __initdata builtin_cmdline[COMMAND_LINE_SIZE] = CONFIG_CMDLINE;
+static const char builtin_cmdline[] __initconst = CONFIG_CMDLINE;
 #else
 static const char builtin_cmdline[] __initconst = "";
 #endif