boot: move package options to a submenu

Change all Kconfig entries for packages in the "Bootloaders" menu to
submenus.

Currently many bootloaders and firmwares in this menu use a 'config'
keyword and thus menuconfig presents their sub-options as a flat list in
the same menu. This is not very convenient as some have a lot of
sub-options, and in (more and more common) configurations with multiple
such packages enabled the menuconfig screen becomes very crowded. Moreover
options of 'string' type don't have a visible indentation when their value
exceeds a few characters, thus making it visually hard to grasp which
package the various options belong to.

Non-bootloader packages having suboptions usually use a 'menuconfig'
keywork to have sub-options in a submenu. Do the same for packages in the
"Bootloaders" menu which have more than one sub-option.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Reviewed-by: Neal Frager <neal.frager@amd.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

boot/syslinux/Config.in

@@ -1,4 +1,4 @@
-config BR2_TARGET_SYSLINUX
+menuconfig BR2_TARGET_SYSLINUX
 	bool "syslinux"
 	depends on BR2_i386 || BR2_x86_64
 	# Make sure at least one of the flavors is installed