linux-4.14.39: Select DELL_RBU as module

The Kconfig help text:

> Say m if you want to have the option of updating the BIOS for your
> DELL system. Note you need a Dell OpenManage or Dell Update package
> (DUP) supporting application to communicate with the BIOS regarding
> the new image for the image update to take effect.  See
> <file:Documentation/dell_rbu.txt> [1] for more details on the driver.

```
$ diff -u /boot/config-4.14.33.mx64.212 /boot/config-4.14.39.mx64.213
--- /boot/config-4.14.33.mx64.212	2018-04-09 15:54:12.000000000
+0200
+++ /boot/config-4.14.39.mx64.213	2018-05-03 12:01:24.000000000
+0200
@@ -1,6 +1,6 @@
 #
 # Automatically generated file; DO NOT EDIT.
-# Linux/x86 4.14.33 Kernel Configuration
+# Linux/x86 4.14.39 Kernel Configuration
 #
 CONFIG_64BIT=y
 CONFIG_X86_64=y
@@ -52,7 +52,7 @@
 CONFIG_INIT_ENV_ARG_LIMIT=32
 CONFIG_CROSS_COMPILE=""
 # CONFIG_COMPILE_TEST is not set
-CONFIG_LOCALVERSION=".mx64.212"
+CONFIG_LOCALVERSION=".mx64.213"
 CONFIG_LOCALVERSION_AUTO=y
 CONFIG_HAVE_KERNEL_GZIP=y
 CONFIG_HAVE_KERNEL_BZIP2=y
@@ -1469,6 +1469,7 @@
 CONFIG_FW_LOADER=y
 CONFIG_FIRMWARE_IN_KERNEL=y
 CONFIG_EXTRA_FIRMWARE=""
+CONFIG_FW_LOADER_USER_HELPER=y
 # CONFIG_FW_LOADER_USER_HELPER_FALLBACK is not set
 CONFIG_ALLOW_DEV_COREDUMP=y
 # CONFIG_DEBUG_DRIVER is not set
@@ -3854,7 +3855,7 @@
 #
 # CONFIG_EDD is not set
 CONFIG_FIRMWARE_MEMMAP=y
-# CONFIG_DELL_RBU is not set
+CONFIG_DELL_RBU=m
 # CONFIG_DCDBAS is not set
 CONFIG_DMIID=y
 # CONFIG_DMI_SYSFS is not set
```

[1] https://www.kernel.org/doc/Documentation/dell_rbu.txt
[2] https://major.io/2016/01/18/updating-dell-poweredge-bios-from-linux/

linux-4.14.39-213.bee

@@ -765,6 +765,7 @@ mee_configure() {
 	CONFIG_RTC_CLASS=y
 	# CONFIG_RTC_INTF_PROC is not set
 	CONFIG_AMD_IOMMU=y
+	CONFIG_DELL_RBU=m
 	CONFIG_GOOGLE_FIRMWARE=y
 	CONFIG_GOOGLE_COREBOOT_TABLE_ACPI=m
 	CONFIG_GOOGLE_MEMCONSOLE_COREBOOT=m