.gitmodules: Activate shallow clone for all submodules

This speeds up the cloning process a lot as only a Git history depth of
1 is used for each submodule, see gitmodules(5) for more information.

Fixes #32

.gitmodules

@@ -1,45 +1,60 @@
 [submodule "core"]
+	shallow = true
 	path = vendor/core
 	url = https://android.googlesource.com/platform/system/core.git
 [submodule "extras"]
+	shallow = true
 	path = vendor/extras
 	url = https://android.googlesource.com/platform/system/extras.git
 [submodule "selinux"]
+	shallow = true
 	path = vendor/selinux
 	url = https://android.googlesource.com/platform/external/selinux.git
 [submodule "f2fs-tools"]
+	shallow = true
 	path = vendor/f2fs-tools
 	url = https://android.googlesource.com/platform/external/f2fs-tools.git
 [submodule "e2fsprogs"]
+	shallow = true
 	path = vendor/e2fsprogs
 	url = https://android.googlesource.com/platform/external/e2fsprogs.git
 [submodule "boringssl"]
+	shallow = true
 	path = vendor/boringssl
 	url = https://boringssl.googlesource.com/boringssl.git
 [submodule "vendor/mkbootimg"]
+	shallow = true
 	path = vendor/mkbootimg
 	url = https://android.googlesource.com/platform/system/tools/mkbootimg
 [submodule "vendor/avb"]
+	shallow = true
 	path = vendor/avb
 	url = https://android.googlesource.com/platform/external/avb
 [submodule "vendor/libbase"]
+	shallow = true
 	path = vendor/libbase
 	url = https://android.googlesource.com/platform/system/libbase
 [submodule "vendor/libziparchive"]
+	shallow = true
 	path = vendor/libziparchive
 	url = https://android.googlesource.com/platform/system/libziparchive
 [submodule "vendor/base"]
+	shallow = true
 	path = vendor/base
 	url = https://android.googlesource.com/platform/frameworks/base
 [submodule "vendor/native"]
+	shallow = true
 	path = vendor/native
 	url = https://android.googlesource.com/platform/frameworks/native
 [submodule "vendor/adb"]
+	shallow = true
 	path = vendor/adb
 	url = https://android.googlesource.com/platform/packages/modules/adb.git
 [submodule "vendor/logging"]
+	shallow = true
 	path = vendor/logging
 	url = https://android.googlesource.com/platform/system/logging.git
 [submodule "vendor/incremental_delivery"]
+	shallow = true
 	path = vendor/incremental_delivery
 	url = https://android.googlesource.com/platform/system/incremental_delivery