core/Makefile: Add ESP to pack-image dependencies

Add the ESP to the dependencies of desktop pack image
for desktop boards setting `PACK_DESKTOP_ESP_IMAGE`.
This avoids having to run m prior to m pack-image for
the product's esp.img.

Bug: 395703174
Test: m pack-image
Flag: EXEMPT desktop only change
Change-Id: I65979ff7b888e85ba5d3788328ef4358ee0c73ed