CONFIG_OF_NET depends on CONFIG_NVMEM in kernel 4.19. To fix some build
problems in mainline Linux kernel CONFIG_NVMEM was changed from tristate
to bool in commit 2a37ce25d9 ("nvmem: disallow modular CONFIG_NVMEM").
This patch in OpenWrt revert the upstream commit and changes
CONFIG_NVMEM back to tristate we just have to make sure to build this in
for all targets which select CONFIG_OF_NET.
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
