toplevel.mk: use relative path for feeds/base symlink

toplevel.mk: use relative path for feeds/base symlink

Currently, feeds/base is created as an absolute symlink to $(TOPDIR)/package.
If the OpenWrt source tree is copied to another location for building (while keeping the old tree), the symlink will still point to the package directory in the old tree.
Using a relative symlink ensures that feeds/base always points to the package directory within the current OpenWrt source tree, improving portability and avoiding incorrect links.

Signed-off-by: Andy Chiang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/20297
Signed-off-by: Robert Marko <[email protected]>