kconfig: Remove redundant $ZEPHYR_BASE from 'source's

The $srctree environment variable is already set to point to the Zephyr
root, so no need to do

    source "$(ZEPHYR_BASE)/Kconfig.zephyr"

in samples. Just

    source "Kconfig.zephyr"

works.

(Things would break if $srctree was set to anything else, because every
'source' in the Kconfig files will be relative to it.)

Also add a 'mainmenu' title to the littlefs sample. It shows up at the
top of menuconfig/guiconfig. Source Kconfig.zephyr instead of Kconfig to
avoid overriding it.

As a sidenote, $(FOO) is better $FOO in Kconfig. $FOO is legacy syntax
that Kconfiglib only supports to be compatible with old Linux kernels.
$(FOO) uses the Kconfig preprocessor.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>