git.sh: support downloading dependency trees

a tree can specify:

tree_depend="treename"

this will make the other tree be downloaded. this is
used for coreboot trees, to ensure that dependency
trees are downloaded, because trees can now re-use
crossgcc from other trees.

Signed-off-by: Leah Rowe <leah@libreboot.org>

config/coreboot/dell/target.cfg

@@ -1,3 +1,4 @@
 tree="dell"
+tree_depend="default"
 xtree="default"
 rev="d862695f5f432b5c78dada5f16c293a4c3f9fce6"

config/coreboot/fam15h_udimm/target.cfg

@@ -1,4 +1,5 @@
 tree="fam15h_udimm"
+tree_depend="fam15h_rdimm"
 xtree="fam15h_rdimm"
 rev="1c13f8d85c7306213cd525308ee8973e5663a3f8"
 payload_grub="n"