base-files: add support for platform_pre_upgrade function

Current NAND sysupgrade process is a bit hard to follow due to the way
of triggering stage1. Currently this is done by leaving a /mark/ in the
form of /tmp/sysupgrade-nand-path during nand_do_platform_check.
Existence of this mark stops standard sysupgrade process (as the result
of sysupgrade_pre_upgrade exit). This may be a bit misleading.

Proposed solution adds a new function that will allow platform.sh
trigger NAND sysupgrade consciously. This will also allow cleaning
nand_do_platform_check limiting it to just checking the image.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 45338