minui: Do not return true from target_has_overlay() if MSM_BSP is not set

Since target_has_overlay() was not ifdefed with MSM_BSP,
it would return true on some devices even if MSM_BSP was unset,
breaking LPM and some other minui functions.

Change-Id: Ic6b524fa0b5e36fc9ffe67d53e2bf33e9d4fd2d3