linux-postmarketos-exynos4: add zstd to makedepends (MR 5569)

Needed by new initramfs compression config settings.

Also remove busybox-static-armv7, we carry a busybox blob in the
kernel repo sources instead.

device/community/linux-postmarketos-exynos4/APKBUILD

@@ -19,7 +19,6 @@ options="!strip !check !tracedeps
 makedepends="
 	bash
 	bison
-	busybox-static-armv7
 	findutils
 	flex
 	gmp-dev
@@ -29,6 +28,7 @@ makedepends="
 	perl
 	postmarketos-installkernel
 	xz
+	zstd
 "
 
 # Source