boot/syslinux: bump version to 6.03

Since 6.03 syslinux release, the gnu-efi sources are bundled whith
syslinux. We need to force the build system to use the Buildroot's
gnu-efi package.

Remove the patch for find-gnu-efi.sh script which was removed, as well
as the SYSROOT variable from the BUILD_CMDS, which is no longer
needed.

There are still some parallel build issues, so MAKE1 can't
be removed for now.

Add a hash file.

Fix indentation.

[Thomas: fix minor typos in a new comment in the .mk file.]

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>