Makefile.lib: find capsule ESL dtsi file with CONFIG_OF_UPSTREAM

When CONFIG_OF_UPSTREAM is enabled, DTS files are in SOC subdirectories (vs the
top level dts directory), but when CONFIG_EFI_CAPSULE_AUTHENTICATE is enabled,
the dynamically created dtsi file containing the capsule ESL DT node is in the
parent directory. This results in a build failure because the #include inserted
in the DTS file is local to the current directory.  Update Makefile to have the
DT preprocessing of #includes search in the parent (dts top level) directory
too.

Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>