devtool: add: delete externalsrc files on npm recipe do_install

The npm class just installs whatever is in ${S}; if you're using
externalsrc in conjunction with it the symlinks (oe-workdir and oe-logs
by default) plus singletask.lock will end up in the final package, which
isn't really right. Introduce a variable so we know the path the files
will be installed into within npm.bbclass, and append to do_install
within the workspace bbappend to delete the files.

(From OE-Core rev: 766845e06db9d7d595e836ea1364c16fa132a413)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/npm.bbclass

@@ -1,6 +1,8 @@
 DEPENDS_prepend = "nodejs-native "
 S = "${WORKDIR}/npmpkg"
 
+NPM_INSTALLDIR = "${D}${libdir}/node_modules/${PN}"
+
 npm_do_compile() {
 	# changing the home directory to the working directory, the .npmrc will
 	# be created in this directory
@@ -14,8 +16,8 @@ npm_do_compile() {
 }
 
 npm_do_install() {
-	mkdir -p ${D}${libdir}/node_modules/${PN}/
-	cp -a ${S}/* ${D}${libdir}/node_modules/${PN}/ --no-preserve=ownership
+	mkdir -p ${NPM_INSTALLDIR}/
+	cp -a ${S}/* ${NPM_INSTALLDIR}/ --no-preserve=ownership
 }
 
 python populate_packages_prepend () {