How to override default Kernel config

[Lei YU]

> After following your instructions, the CONFIG_DEVMEM is still not set in the below '.config'
>
> workspace/sources/linux-aspeed/.config
>
> It did work if I directly modified meta-openbmc-bsp/meta-aspeed/recipes-kernel/linux/linux-aspeed/ast2500/defconfig

Then you need to make sure if your build config include the machine layer by
checking the build configuration.

E.g. below is meta-romulus' configuration, check if your meta-machine is
correctly included.

    Build Configuration:
    ...
    meta
    meta-poky
    meta-oe
    meta-networking
    meta-perl
    meta-python
    meta-webserver
    meta-virtualization
    meta-phosphor
    meta-aspeed
    meta-openpower
    meta-ibm
    meta-romulus   <== Your meta-machine should appear here.

Btw, what is your machine, is it in OpenBMC tree?