.gitignore: fix entries

Some entries were matching on subfolders.

Signed-off-by: Tim Niemeyer <tim@tn-x.org>
Reviewed-by: Tobias Klaus <tk+ff@meskal.net>

.gitignore

-selected_bsp
-selected_community
-build
-bin
-src/dl
-src/openwrt
-src/packages/fastd
-src/packages/openwrt
-src/packages/batman_adv
-src/packages/routing
-.project
-./*.patch
+/selected_bsp
+/selected_community
+/build*
+/bin*
+/src/dl
+/src/openwrt
+/src/packages/fastd
+/src/packages/openwrt
+/src/packages/batman_adv
+/src/packages/routing
+/.project
+/*.patch