kernel: bump 5.10 to 5.10.20

Also add a new kconfig symbol (CONFIG_KCMP) to the generic config,
disabling the SYS_kcmp syscall (it was split from
CONFIG_CHECKPOINT_RESTORE, which is disabled by default, so the
previous behaviour is kept).

Removed (upstreamed) patches:
  070-net-icmp-pass-zeroed-opts-from-icmp-v6-_ndo_send-bef.patch
  081-wireguard-device-do-not-generate-ICMP-for-non-IP-pac.patch
  082-wireguard-queueing-get-rid-of-per-peer-ring-buffers.patch
  083-wireguard-kconfig-use-arm-chacha-even-with-no-neon.patch
  830-v5.12-0002-usb-serial-option-update-interface-mapping-for-ZTE-P685M.patch

Manually rebased patches:
  313-helios4-dts-status-led-alias.patch
  104-powerpc-mpc85xx-change-P2020RDB-dts-file-for-OpenWRT.patch

Run tested:
  ath79 (TL-WDR3600)
  mvebu (Turris Omnia)

Signed-off-by: Rui Salvaterra <rsalvaterra@gmail.com>

target/linux/bcm63xx/patches-5.10/401-bcm963xx_real_rootfs_length.patch

@@ -15,7 +15,7 @@
  	/* 240-255: Unused at present */
 --- a/drivers/mtd/parsers/parser_imagetag.c
 +++ b/drivers/mtd/parsers/parser_imagetag.c
-@@ -132,7 +132,8 @@ static int bcm963xx_parse_imagetag_parti
+@@ -136,7 +136,8 @@ static int bcm963xx_parse_imagetag_parti
  		} else {
  			/* OpenWrt layout */
  			rootfsaddr = kerneladdr + kernellen;