boot/syslinux: install in a sub-dir of $(BINARIES_DIR)

Since syslinux can now install quite a number of files, install
them in a sub-directory of $(BINARIES_DIR) for clarity.

It also aligns it to rpi-firmware, grub2, gummiboot, that install
all of their files in a sub-dir of $(BINARIES_DIR), too.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>