rauc: generate RAUC configuration file from template

As we add support for additional boards, doing it by copying and
modifying a file with relatively few parts changing got tedious.

The change adds three new variables - RAUC_{COMPAT,SLOT_A,SLOT_B},
and defines them for the boards supported thus far.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>