kernel: Make CONFIG_NVMEM tristate

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>

target/linux/generic/hack-4.19/290-nvmem-make-CONFIG_NVMEM-tristate-again.patch

@@ -0,0 +1,33 @@
+From 2579d9b982c7232f9354bcca5262e26a84c38799 Mon Sep 17 00:00:00 2001
+From: Hauke Mehrtens <hauke@hauke-m.de>
+Date: Fri, 2 Nov 2018 17:40:32 +0100
+Subject: [PATCH] nvmem: make CONFIG_NVMEM tristate again
+
+Only build it in when OF_NET is selected and make it possible to build
+it as module otherwise.
+
+Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
+---
+ drivers/nvmem/Kconfig | 2 +-
+ drivers/of/Kconfig    | 1 +
+ 2 files changed, 2 insertions(+), 1 deletion(-)
+
+--- a/drivers/nvmem/Kconfig
++++ b/drivers/nvmem/Kconfig
+@@ -1,5 +1,5 @@
+ menuconfig NVMEM
+-	bool "NVMEM Support"
++	tristate "NVMEM Support"
+ 	help
+ 	  Support for NVMEM(Non Volatile Memory) devices like EEPROM, EFUSES...
+ 
+--- a/drivers/of/Kconfig
++++ b/drivers/of/Kconfig
+@@ -71,6 +71,7 @@ config OF_IRQ
+ 
+ config OF_NET
+ 	depends on NETDEVICES
++	select NVMEM
+ 	def_bool y
+ 
+ config OF_MDIO