kernel: Activate CONFIG_OPTIMIZE_INLINING

kernel: Activate CONFIG_OPTIMIZE_INLINING

This will reduce the size of the kernel if CONFIG_CC_OPTIMIZE_FOR_SIZE is
set like for all targets with small_flash feature flag.
I haven't seen any changes for an ARM64 target which optimizes the
kernel for speed instead.

On the ath79/tiny target the uncompressed kernel size was reduced by
3.2% and the compressed kernel size by 2.1%

kernel size with CONFIG_OPTIMIZE_INLINING=n
4346412 build_dir/target-mips_24kc_musl/linux-ath79_tiny/vmlinux
1391169 build_dir/target-mips_24kc_musl/linux-ath79_tiny/tplink_tl-wr941-v4-kernel.bin

Kernel size with CONFIG_OPTIMIZE_INLINING=y
4212396 build_dir/target-mips_24kc_musl/linux-ath79_tiny/vmlinux
1362051 build_dir/target-mips_24kc_musl/linux-ath79_tiny/tplink_tl-wr941-v4-kernel.bin

This change is currently pending for kernel 5.2 and already in
linux-next, this updates our patch to match the upstream version.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit 6dac1c0a9b94b62b6412b74a8997f728570f36be)