bootimg_pcbios: move syslinux funcs to end of file

bootimg_pcbios: move syslinux funcs to end of file

This commit moves the seperated syslinux creation
functions from their current position to end of
file in the order

	1. _do_configure_syslinux
	2. _do_prepare_syslinux
	3. _do_install_syslinux

This is to prepare for inclusion of other
bootloaders. It also makes reading
through the wics plugin much easier if
you group bootloader specific partition
creation functions together and place them
at the bottom of the file versus leaving
them in their current position.

Signed-off-by: Vincent Davis Jr. <vince@underview.tech>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>