boot/syslinux: fix build with EFI support

Commit 1a437fd2 bumped gnu-efi to version 3.0.9. This breaks the build
of syslinux with EFI support due to multiple definitions of 'memset' and
'memcpy'. Backport a patch already applied upstream to fix the problem.

Signed-off-by: Carlos Santos <casantos@datacom.com.br>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>