pbl: Fix typo in make causing pbl files to be rebuilt

We had pbl_cc__o_c instead of pbl_cc_o_c. This caused all pbl object
files to be rebuilt everytime due to missing .cmd file.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
ARM: pbl: fix make dependencies

The pbl used 'zbarebox.bin' as target instead of the real file. This
lead to strange effects that the images depending on zbarebox.bin were
only built every second time. This uses the full path as target.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
make: Add barebox_default_env.* to targets

Otherwise it gets rebuilt everytime.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
ARM: zynq: Add cmd to generate zynq image

Add a command to generate the zynq image instead of generating it
directly. This causes a rebuild exactly when necessary and prints
a "ZYNQ-IMG" to the commandline during compilation.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>