bitbake: data_smart: Fix multiple override interaction with append and prepend operators

bitbake: data_smart: Fix multiple override interaction with append and prepend operators

Variables which used multiple overrides and the append/prepend operators were
not functioning correctly. This change fixes that.

This fixes the testcase:

OVERRIDES = "linux:x86"
TESTVAR = "original"
TESTVAR_append_x86 = " x86"
TESTVAR_append_x86_linux = " x86+linux"
TESTVAR_append_linux_x86 = " linux+x86"

[YOCTO #2672]

(Bitbake rev: dc35a2e506e15fb7ddbf74c3b3280e9e83ab33bb)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>