clockwwork recovery build fixes
fix BOARD_CUSTOM_BOOTIMG_MK
Fix mka for some weird configurations.
Some make versions identify "make -j N" calls as "make N -j <?>" instead
of "make -jN" it was intended to be (a.k.a. the "make 8" error). This
makes sure make -jN is called and also replaces backticks with $() just
for code readability.

Change-Id: I3487a8c92e07cd3a4e07e2642df620ed234516e1