boot/syslinux: add missing python dependency

Syslinux use some python scripts during the build and they
are using python interpreter by default. It fail to build
when there is no python interpreter on the host.

[...]/syslinux-6.03/com32/cmenu/menugen.py
make[6]: python: No such file or directory

Since Syslinux 5.00, we can override the python interpreter
used during the build:

https://repo.or.cz/syslinux.git/commitdiff/4dec62ce9c2c0d170f21b3ae2d7c618eb7a30c05

Add the	missing	host-python3 dependency and override
it in SYSLINUX_BUILD_CMDS.

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/1614446766

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
[yann.morin.1998@free.fr: fix check-package]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit 6ccfd40711e122c720145044eb4d145a8bacd9db)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/syslinux/syslinux.mk

@@ -14,7 +14,12 @@ SYSLINUX_LICENSE_FILES = COPYING
 SYSLINUX_INSTALL_IMAGES = YES
 
 # host-util-linux needed to provide libuuid when building host tools
-SYSLINUX_DEPENDENCIES = host-nasm host-upx util-linux host-util-linux
+SYSLINUX_DEPENDENCIES = \
+	host-nasm \
+	host-python3 \
+	host-upx \
+	host-util-linux \
+	util-linux
 
 ifeq ($(BR2_TARGET_SYSLINUX_LEGACY_BIOS),y)
 SYSLINUX_TARGET += bios
@@ -59,6 +64,7 @@ define SYSLINUX_BUILD_CMDS
 		CC_FOR_BUILD="$(HOSTCC)" \
 		CFLAGS_FOR_BUILD="$(HOST_CFLAGS)" \
 		LDFLAGS_FOR_BUILD="$(HOST_LDFLAGS)" \
+		PYTHON=$(HOST_DIR)/bin/python3 \
 		$(SYSLINUX_EFI_ARGS) -C $(@D) $(SYSLINUX_TARGET)
 endef