configs/sheevaplug_defconfig: add hashes for u-boot and Linux

And enable BR2_DOWNLOAD_FORCE_CHECK_HASHES.

Generated by utils/add-custom-hashes, with the (redundant)
linux-headers.hash replaced by a symlink.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 56ce35d235b7f7b297e33c118e4e7b64491f73c6)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

board/sheevaplug/patches/linux-headers/linux-headers.hash

@@ -0,0 +1 @@
+../linux/linux.hash

board/sheevaplug/patches/linux/linux.hash

@@ -0,0 +1,2 @@
+# Locally calculated
+sha256  0820fdb7971c6974338081c11fbf2dc869870501e7bdcac4d0ed58ba1f57b61c  linux-4.14.336.tar.xz

board/sheevaplug/patches/uboot/uboot.hash

@@ -0,0 +1,2 @@
+# Locally calculated
+sha256  9f10df88bc91b35642e461217f73256bbaeeca9ae2db8db56197ba5e89e1f6d4  u-boot-2018.07.tar.bz2

configs/sheevaplug_defconfig

@@ -4,6 +4,7 @@ BR2_arm926t=y
 
 # system
 BR2_GLOBAL_PATCH_DIR="board/sheevaplug/patches"
+BR2_DOWNLOAD_FORCE_CHECK_HASHES=y
 BR2_TARGET_GENERIC_GETTY=y
 BR2_TARGET_GENERIC_GETTY_PORT="ttyS0"
 BR2_SYSTEM_DHCP="eth0"