package/systemd: enable tmpfs kernel option

systemd automatically creates tmpfs mounts for /tmp, /run, /dev/shm, and
others. If tmpfs is not enabled in the kernel, systemd crashes with an
unknown filesystem error on boot. This hasn't manifested itself during
tests with defconfigs such as pc_x86_64_efi_defconfig for a few reasons:

  - No defconfig uses systemd as the init system by default
  - Even if a user does use pc_x86_64_efi_defconfig and changes to systemd,
    the board/pc/linux.config file enables CONFIG_PCI, which selects
    CONFIG_TMPFS.
  - The systemd init tests use a prebuilt kernel with tmpfs enabled.

Enable tmpfs unconditionally to ensure that systemd always boots, even with
a minimal kernel built.

Signed-off-by: Adam Duskett <adam.duskett@amarulasolutions.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

package/systemd/systemd.mk

@@ -847,6 +847,7 @@ SYSTEMD_CONF_ENV = $(HOST_UTF8_LOCALE_ENV)
 SYSTEMD_NINJA_ENV = $(HOST_UTF8_LOCALE_ENV)
 
 define SYSTEMD_LINUX_CONFIG_FIXUPS
+	$(call KCONFIG_ENABLE_OPT,CONFIG_TMPFS)
 	$(call KCONFIG_ENABLE_OPT,CONFIG_DEVTMPFS)
 	$(call KCONFIG_ENABLE_OPT,CONFIG_CGROUPS)
 	$(call KCONFIG_ENABLE_OPT,CONFIG_INOTIFY_USER)