Add build/install scripts and improve README.

scripts/build.sh

@@ -0,0 +1,13 @@
+#!/bin/bash
+
+STATIC="$1"
+
+if [ -z "$STATIC" ]; then
+    STATIC=0
+fi
+
+if [ -z "$ROOT" ]; then
+    LIBBPF_LIBXDP_STATIC=$STATIC make
+else
+    cd $ROOT && LIBBPF_LIBXDP_STATIC=$STATIC make
+fi

scripts/clean.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [ -z "$ROOT" ]; then
+    make clean
+else
+    cd $ROOT && make clean
+fi

scripts/install.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [ -z "$ROOT" ]; then
+    make install
+else
+    cd $ROOT && make install
+fi

scripts/libxdp_build.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [ -z "$ROOT" ]; then
+    make libxdp
+else
+    cd $ROOT && make libxdp
+fi

scripts/libxdp_clean.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [ -z "$ROOT" ]; then
+    make libxdp_clean
+else
+    cd $ROOT && make libxdp_clean
+fi

scripts/libxdp_install.sh

@@ -0,0 +1,7 @@
+#!/bin/bash
+
+if [ -z "$ROOT" ]; then
+    make libxdp_install
+else
+    cd $ROOT && make libxdp_install
+fi