Revert "poco: fix parallel build failure"

This reverts commit 32f4957b.

Adding the phony objdir target as a prerequisite of all the .o targets
doesn't work, as they are then always considered out of date, leading
to poco getting rebuilt (without the proper MYSQL_{INC,LIB}DIR settings)
at staging-install / target-install time.

Instead simply use MAKE1 for the build step.

Fixes http://autobuild.buildroot.net/results/1cca8b4115674f12884bcbc0c680efed0fb939ae



Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>