mkbootimg: support longer kernel command lines

We can't simply increase the value of BOOT_ARGS_SIZE since that
would break past binary compatibility; the offset of 'id' would
change. This can cause serious problems with incremental OTA updates.

Instead, define a supplemental field which stores command line
information beyond the first 512 bytes. A value of 1024 was chosen
to keep the total size of struct boot_img_hdr under the smallest
page size of 2048 bytes.

Even if the extra_cmdline field is used, the original cmdline
field is still always NULL-terminated to avoid issues with older
bootloaders that haven't been updated.

Change-Id: I887e6f1db351a5b71a61e3a03363cf8856123d74
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>