PHAL: Add service dependency to DEVTREE creation

Start the services that require DEVTREE after the
openpower-update-bios-attr-table.service since that's the one that
sets up the DEVTREE symlink. Also update the path to DEVTREE to the one
defined by the meson options.

Tested: Verified phal started after the bios attr service file, powered
on, and issued reboot at power on without any errors. Verified service
files had devtree path of:
Environment="PDBG_DTB=/media/hostfw/running/DEVTREE"

Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
Change-Id: If41f86b287b70c48d17c6f4b2af9aa0adfb81488
diff --git a/meson.build b/meson.build
index 889109d..fddea45 100644
--- a/meson.build
+++ b/meson.build
@@ -61,6 +61,9 @@
 unit_subs = configuration_data()
 unit_subs.set('bindir', join_paths(get_option('prefix'), get_option('bindir')))
 unit_subs.set('ENABLE_PHAL_TRUE', '#')
+unit_subs.set('CEC_DEVTREE_RW_PATH', get_option('CEC_DEVTREE_RW_PATH'),
+               description : 'Path to the devtree file r/w version'
+             )
 
 if get_option('phal').enabled() and get_option('p9').enabled()
     error('phal and p9 cannot be selected at the same time')