Make dump-many-vars work for large variables

When printing a large variable like PRODUCT_PACKAGES,
dump-many-vars could fail because the command line is
too long. This is seen as a E2BIG error on the execve
in kati.

To solve this, split the printing up by word in the
variable.

Change-Id: I49db726d7a4769a2f02028194d79934ed5655adf
Fixes: 172287533
Test: lunch bonito-userdebug; get_build_var PRODUCT_PACKAGES