kernel: remove 3.12 support since none of the targets is using it

include/kernel-version.mk

target/linux/generic/config-3.12

...s-3.12/009-mtd_m25p80_add_support_for_esmt_f25l32pa.patch

target/linux/generic/patches-3.12/020-ssb_update.patch

target/linux/generic/patches-3.12/025-bcma_backport.patch

target/linux/generic/patches-3.12/060-hso_devices.patch

...eric/patches-3.12/061-arm_xz_decompressor_build_fix.patch

...eneric/patches-3.12/062-mips_decompressor_build_fix.patch

target/linux/generic/patches-3.12/100-overlayfs.patch

...t/linux/generic/patches-3.12/102-ehci_hcd_ignore_oc.patch

...s-3.12/120-bridge_allow_receiption_on_disabled_port.patch

...ric/patches-3.12/130-mips_remove_plat_dma_functions.patch

...ric/patches-3.12/131-improve_noncoherent_dma_checks.patch

.../linux/generic/patches-3.12/132-mips_inline_dma_ops.patch

target/linux/generic/patches-3.12/200-fix_localversion.patch

...t/linux/generic/patches-3.12/201-extra_optimization.patch

...t/linux/generic/patches-3.12/202-reduce_module_size.patch

...inux/generic/patches-3.12/203-kallsyms_uncompressed.patch

target/linux/generic/patches-3.12/204-module_strip.patch

...nux/generic/patches-3.12/210-darwin_scripts_include.patch