ANDROID: gki_defconfig: enabled CONFIG_TMPFS explicitly

otherwise will cause init boot failure reported like this:

[    1.959344] Run /init as init process
[    1.966119] tmpfs: Unknown parameter 'mode'
[    1.971092] tmpfs: Unknown parameter 'mode'
[    1.975426] tmpfs: Unknown parameter 'mode'
[    1.979709] tmpfs: Unknown parameter 'mode'
[    1.984298] init: mount("tmpfs", "/dev", "tmpfs", MS_NOSUID, "mode=0755") failed Invalid argument
[    1.993214] init: mount("tmpfs", "/mnt", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV, "mode=0755,uid=0,gid=1000") failed Invalid argument
[    2.005417] init: mount("tmpfs", "/apex", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV, "mode=0755,uid=0,gid=0") failed Invalid argument
[    2.017435] init: mount("tmpfs", "/debug_ramdisk", "tmpfs", MS_NOEXEC | MS_NOSUID | MS_NODEV, "mode=0755,uid=0,gid=0") failed Invalid argument
[    2.030276] init: Init encountered errors starting first stage, aborting
[    2.057074] init: #00 pc 00000000002eeac8  /init (UnwindStackCurrent::UnwindFromContext(unsigned long, void*)+96)
[    2.067403] init: #01 pc 00000000002730c4  /init (android::init::InitFatalReboot()+92)
[    2.075341] init: #02 pc 00000000002754f4  /init (android::init::InitAborter(char const*)+44)
[    2.083887] init: #03 pc 00000000002ae4f8  /init (android::base::LogMessage::~LogMessage()+608)
[    2.092605] init: #04 pc 000000000026aba8  /init (android::init::FirstStageMain(int, char**)+4400)
[    2.101588] init: #05 pc 00000000003599e4  /init (__real_libc_init(void*, void (*)(), int (*)(int, char**, char**), structors_array_t const*, bionic_tcb*)+572)
[    2.115863] init: Reboot ending, jumping to kernel
[    2.121709] reboot: Restarting system with command 'bootloader'

Tested: with hikey

Change-Id: I607856cbcc5bfdaf304b13089c07ca0cf10c2f76
Signed-off-by: Yongqin Liu <yongqin.liu@linaro.org>