imagebuilder: fix broken image generation with external targets

imagebuilder: fix broken image generation with external targets

When using external targets there is a symlink being created for the
target under target/linux which then becomes dangling under Image
Builder. Fix it by dereferencing the possible symlink.

Tested on IB with external target, ipq40xx and mvebu.

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit 621f39d1f438bf95dbae667c575926fa16a6d797)
(cherry picked from commit ec9af870f3278f75549836b469baefa260e2ed41)
(cherry picked from commit 3008f1f441a41e162311cee1ccadfdaaec1581c1)