apm821xx: disable NVMEM_U_BOOT_ENV
The main point of it currently is to extract mac addresses. That is not
being done as MAC addresses are elsewhere.
Disable it until it becomes more feature packed and there's an actual
use for it.
All devices already have config definitions. NVMEM prevents redundant
support as well as write support.
Signed-off-by: Rosen Penev <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/16618
Signed-off-by: Robert Marko <[email protected]>
Rosen Penev