boot/shim: security bump to version 15.6

Fixes the following security issue:

CVE-2022-28737: There's a possible overflow in handle_image() when shim
tries to load and execute crafted EFI executables

https://github.com/advisories/GHSA-hmxr-46w2-jjwh

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

boot/shim/shim.hash

@@ -1,3 +1,3 @@
 # locally computed hash
-sha256  8344473dd10569588b8238a4656b8fab226714eea9f5363f8c410aa8a5090297  shim-15.4.tar.bz2
+sha256  eab91644a3efe91a666399f5d8eb3eed0e04d04f79d4b6c0b278ef7747a239a5  shim-15.6.tar.bz2
 sha256  15edf527919ddcb2f514ab9d16ad07ef219e4bb490e0b79560be510f0c159cc2  COPYRIGHT

boot/shim/shim.mk

@@ -4,7 +4,7 @@
 #
 ################################################################################
 
-SHIM_VERSION = 15.4
+SHIM_VERSION = 15.6
 SHIM_SITE = https://github.com/rhboot/shim/releases/download/$(SHIM_VERSION)
 SHIM_SOURCE = shim-$(SHIM_VERSION).tar.bz2
 SHIM_LICENSE = BSD-2-Clause