image: introduce genimg_get_kernel_addr()

Kernel address is normally stored as a string argument of bootm or bootz.
This function is taken out from boot_get_kernel() of bootm.c, which can be
reused by others.

Signed-off-by: Bryan Wu <pengw@nvidia.com>
[trini: Fix warnings with CONFIG_FIT]
Signed-off-by: Tom Rini <trini@ti.com>