wic/bootimg-pcbios.py: checking the existance of syslinux

wic/bootimg-pcbios.py: checking the existance of syslinux

While syslinux not existed in $bootimg_dir, there was a error:

$ wic create directdisk -e core-image-minimal
...
|Creating image(s)...
|Error: exec_cmd: install -m 444 /home/jiahongxu/yocto/
build-20141010-yocto/tmp/deploy/images/qemux86-64/syslinux/
ldlinux.sys /var/tmp/wic/build/hdd/boot/ldlinux.sys
returned '1' instead of 0
...

Add checking for the existance of syslinux to fix this issue.
If syslinux didn't exist in anywhere, prompt user to build it.

[YOCTO #6826]

(From OE-Core rev: c2d4364d9fbbda64598f0a3eb0b0890932bb7742)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>