sh: Allow for kernel command line concatenation.

So far kernel command line arguments could be passed in by a bootloader
or defined as CONFIG_CMDLINE, which completely overwriting the first one.

This change allows a developer to declare selected kernel parameters in
a kernel configuration (eg. project-specific defconfig), retaining
possibility of passing others by a bootloader.

The obvious examples of the first type are MTD partition or
bigphysarea-like region definitions, while "debug" option or network
configuration should be given by a bootloader or a JTAG boot script.

Signed-off-by: Pawel Moll <pawel.moll@st.com>
Signed-off-by: Stuart Menefy <stuart.menefy@st.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>