linux-purism-librem5: upgrade to 5.11.4 (MR 2018)

Upstream change logs:

5.11.2:
https://source.puri.sm/Librem5/linux-next/-/commit/fdc91c4e20af5146c9733f16ca6912fb5721e69b

5.11.4:
https://source.puri.sm/Librem5/linux-next/-/commit/a178aa515c9474d8a3d4b473def3027be96d6772

This also adds a -dev subpackage, and cleans up makedepends

[ci:skip-build]: already built successfully in CI

device/community/linux-purism-librem5/APKBUILD

@@ -1,7 +1,7 @@
 # Reference: <https://postmarketos.org/vendorkernel>
 # Maintainer:  Clayton Craft <clayton@craftyguy.net>
 pkgname=linux-purism-librem5
-pkgver=5.11.1
+pkgver=5.11.4
 pkgrel=0
 _purismrel=1
 # <kernel ver>.<purism kernel release>
@@ -14,20 +14,17 @@ url="https://source.puri.sm/Librem5/linux-next"
 license="GPL-2.0-only"
 options="!strip !check !tracedeps pmb:cross-native"
 makedepends="
-	bash
-	bc
 	bison
 	devicepkg-dev
-	elfutils-dev
 	findutils
 	flex
-	gmp-dev
 	installkernel
-	linux-headers
 	openssl-dev
 	perl
-	sed
+	rsync
+	xz
 	"
+subpackages="$pkgname-dev"
 
 # Source
 _repository="linux-next"
@@ -63,7 +60,19 @@ package() {
 		INSTALL_DTBS_PATH="$pkgdir/usr/share/dtb"
 
 }
-sha512sums="fb6c00ac3675a0702fc25ac8fa1fee41479218ee9e9b10e7a22f48475c9b1bc600f9f238e4a8dad656c929c2b6cea11918aee557f49af6f7041d344e345759d1  linux-purism-librem5-5.11.1pureos1.tar.gz
+
+dev() {
+	provides="linux-headers"
+	replaces="linux-headers"
+
+	cd $builddir
+
+	# https://github.com/torvalds/linux/blob/master/Documentation/kbuild/headers_install.rst
+	make -j1 headers_install \
+		ARCH="$_carch" \
+		INSTALL_HDR_PATH="$subpkgdir"/usr
+}
+sha512sums="d2e0f49403413f581aee6e47e021cb04fc003fbf821349c188a0c6a57561ba702c327badf9b181c6f8fdc6952ebc7f061482d6508ae48dc336842eb94c5ed558  linux-purism-librem5-5.11.4pureos1.tar.gz
 9870bff4b187188b519b23264c2634ee4232011fed6d2f66a7b4971db354ac3dffa0e1552bd0dc953c66ec622e18ce8899fdbcfba94f60867fc5004d6da96753  8f11380ec32912370b8ae9134a0387a6f18862f7.patch
 5baae99010bde62e253fdd56f21ba096c217ba2ab9c367c80b34bc0f445a79a8fb8b5d14682f71ad6061d73c81fc16a608f4be037d792978dbbaf74267844260  0001-Revert-arm64-dts-librem5-Drop-separte-DP-device-tree.patch
-329d4158fea4859a67903880bef118cdde0ef1bc6fab616494bb00f760478d0ab20a1d7a262cf370fa6c5ad390b2807c05159a0b16a85464ff8de1d38dd39815  config-purism-librem5.aarch64"
+d27cc078f49563fa935d104846017c9ba28e859d5231d202c68b6438e25f381747206fe0be86d323bd50dc457e67b02a52eb60a845c35bd33d7f2367893781d2  config-purism-librem5.aarch64"