build: Fix check on BOARD_CUSTOM_BOOTIMG_MK

In the current code, '!' is added as prefix to the string contained in
$(BOARD_CUSTOM_BOOTIMG_MK). This means that !$(BOARD_CUSTOM_BOOTIMG_MK)
is never an empty string and therefore the condition is always true.

The if function has a third optional argument that is evalueted when the
condition is false, so use it to fix the current code.

Change-Id: If1e459f6c40f655b47a12b77761921c943964e3b
1 file changed