kexec breaks with 5.4 due to memzero_explicit

[Arvind Sankar]

Hi, arch/x86/purgatory/purgatory.ro has an undefined symbol
memzero_explicit. This has come from commit 906a4bb97f5d ("crypto:
sha256 - Use get/put_unaligned_be32 to get input, memzero_explicit")
according to git bisect.

The patch mentions that it impacts purgatory code, but I don't see any
changes to actually include the definition of memzero_explicit into
purgatory? It used to get memset from arch/x86/boot/compressed/string.c
I think.