boot/syslinux: fix gcc-10.x compile

boot/syslinux: fix gcc-10.x compile

Add two patches ([1], [2]) taken from the fedora syslinux package ([3]) to fix
compile/linking with gcc-10.x compiler.

[1] https://src.fedoraproject.org/rpms/syslinux/raw/rawhide/f/0005-Workaround-multiple-definition-of-symbol-errors.patch
[2] https://src.fedoraproject.org/rpms/syslinux/raw/rawhide/f/0006-Replace-builtin-strlen-that-appears-to-get-optimized.patch
[3] https://src.fedoraproject.org/rpms/syslinux/tree/rawhide

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>