configs/common: enable KERNEL_DEVMEM (/dev/mem)

This is required for rainbow on Turris 1.x.

It is handy in general for any tool that needs to access memory mapped
devices or memory directly in general so it is enabled on all devices.

configs/common/kernel

@@ -2,5 +2,8 @@
 CONFIG_KERNEL_DEVTMPFS=y
 CONFIG_KERNEL_DEVTMPFS_MOUNT=y
 
+# Provide /dev/mem
+CONFIG_KERNEL_DEVMEM=y
+
 # Enable ACL on user's request
 CONFIG_USE_FS_ACL_ATTR=y