kernel/3.10: add new Kconfig option for firmware partition split

Add a new kernel config option for generic firmware partition
split support and change the uImage split support to depend on
the new option. Aslo rename the MTD_UIMAGE_SPLIT_NAME option to
MTD_SPLIT_FIRMWARE_NAME to make it more generic.

The patch is in preparation for multiple firmware format
support.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 38002