introduce buildcfg module and PROXMOX_CONFIGDIR

buildcfg.rs should contain convenience variables or macros
for using build-time configured variables

For now we replace hardcoded "/etc/proxmox-backup/<foo>"
with configdir!("<foo>").

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>

Makefile

@@ -33,6 +33,7 @@ export PROXMOX_PKG_RELEASE=${PKGREL}
 export PROXMOX_PKG_REPOID=${GITVERSION}
 
 export PROXMOX_JSDIR := $(JSDIR)
+export PROXMOX_CONFIGDIR := $(SYSCONFDIR)/proxmox-backup
 
 DEB=${PACKAGE}_${PKGVER}-${PKGREL}_${ARCH}.deb
 DSC=${PACKAGE}_${PKGVER}-${PKGREL}.dsc