bootimage: Use ${S} explicitly for generated config files

The syslinux and grub-efi classes were generating config files in the current
working directory. This caused a failure due to a race in the creation of the
directories leading to cwd changing and the build failing to find the config
files. While this has been addressed in bitbake, it is better to use an
explicit path.

While ${WORKDIR} may seem a more appropriate place, the recipe
already uses ${S} for the "hdd" and "cd" construction, so we use ${S}
here to keep things consolidated and consistent and address the issue
with minimal change.

(From OE-Core rev: 2c4b1675eb3c8c599e3b7f33e459aa608c2b93f2)

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/syslinux.bbclass

@@ -15,8 +15,8 @@
 do_bootimg[depends] += "syslinux:do_populate_sysroot \
                         syslinux-native:do_populate_sysroot"
 
-SYSLINUXCFG  = "syslinux.cfg"
-SYSLINUXMENU = "menu"
+SYSLINUXCFG  = "${S}/syslinux.cfg"
+SYSLINUXMENU = "${S}/menu"
 
 SYSLINUX_ISODIR = "${ISODIR}/isolinux"
 SYSLINUX_HDDDIR = "${HDDDIR}"