brcm2708: remove linux 3.18 support

target/linux/brcm2708/bcm2708/config-3.18

target/linux/brcm2708/bcm2709/config-3.18

.../brcm2708/patches-3.18/0001-Main-bcm2708-linux-port.patch

.../brcm2708/patches-3.18/0002-Add-bcm2708_gpio-driver.patch

...t/linux/brcm2708/patches-3.18/0003-Add-quick-config.patch

...linux/brcm2708/patches-3.18/0004-Add-dwc_otg-driver.patch

.../brcm2708/patches-3.18/0005-bcm2708-watchdog-driver.patch

...dd-non-mainline-source-for-rtl8192cu-wireless-drive.patch

...007-Speed-up-console-framebuffer-imageblit-function.patch

...2708/patches-3.18/0008-fbdev-add-FBIOCOPYAREA-ioctl.patch

...cm2708/patches-3.18/0009-bcm2708-framebuffer-driver.patch

...nux/brcm2708/patches-3.18/0010-bcm2708-vchiq-driver.patch

...1-vchiq-Avoid-high-load-when-blocked-and-unkillable.patch

....18/0012-cma-Add-vc_cma-driver-to-enable-use-of-CMA.patch

...rcm2708/patches-3.18/0013-bcm2708-alsa-sound-driver.patch

...4-Add-hwrng-hardware-random-number-generator-driver.patch

...s-3.18/0015-lirc-added-support-for-RaspberryPi-GPIO.patch

...linux/brcm2708/patches-3.18/0016-Add-cpufreq-driver.patch

...dded-hwmon-thermal-driver-for-reporting-core-temper.patch

...s-3.18/0018-Allow-mac-address-to-be-set-in-smsc95xx.patch