ARM: keystone2: move the custom build rules out to keystone specific makefile

Keystone has build rules introduced by commit ef509b90 ("k2hk: add
support for k2hk SOC and EVM") and commit 0e7f2dba ("keystone: add
support for NAND gpheader image").

These are not reused by other platforms for the build, hence there is no
clear benefit is maintaining them in the generic makefile as a build
target. move these to the keystone specific make option

Original idea of using config.mk by Lokesh Vutla <lokeshvutla@ti.com>

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>