add generic builtin command line

add generic builtin command line

This code allows architectures to use a generic builtin command line.
The state of the builtin command line options across architecture is
diverse. On x86 and mips they have pretty much the same code and the
code prepends the builtin command line onto the boot loader provided
one. On powerpc there is only a builtin override and nothing else.

The code in this commit unifies the mips and x86 code into a generic
header file under the CONFIG_GENERIC_CMDLINE option. When this
option is enabled the architecture can call the cmdline_add_builtin()
to add the builtin command line.

[maksym.kokhan@globallogic.com: fix cmdline_add_builtin() macro]
Cc: Daniel Walker <dwalker@fifo99.com>
Cc: Daniel Walker <danielwa@cisco.com>
Cc: xe-linux-external@cisco.com
Signed-off-by: Daniel Walker <danielwa@cisco.com>
Signed-off-by: Maksym Kokhan <maksym.kokhan@globallogic.com>