build: fix stale package install stamps for unselected variants
When a package variant changes from =y to =m, its .install stamp file
was not being cleaned up, causing both variants to appear in the
package install list.
Fix by making compile depend on prepare-package-install, ensuring
the stamp file is always cleared. Only =y packages recreate it.
Signed-off-by: Felix Fietkau <[email protected]>
Felix Fietkau