sparse: add as a new package selectable from the config

This change does multiple things, all related to enable sparse usage as
a static analysis tool selectable from the OpenWrt configuration:
*add a KERNEL_SPARSE option in the config to add sparse to the kernel
 build (through the C=1 option usage)
*add sparse as a new host tools. It will get selected automatically when
 the above option will be enabled

Signed-off-by: Mathieu Olivari <mathieu@qca.qualcomm.com>

SVN-Revision: 40490