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