of: fix CONFIG_CMDLINE_EXTEND

strlcat takes the size of the buffer, not the number of characters
to concatenate.  If the size of the device tree command line p is
larger than the CONFIG_CMDLINE string data, then strcat(data, p, l)
will hit a BUG_ON because strlen(data) > l.

Replace the second strlcat with a strncpy plus a manual null
termination.

Also rearrange the code to reduce indent depth to make it more
readable, and replace data with a char *cmdline to avoid extra
casts.

Signed-off-by: Colin Cross <ccross@android.com>