Fix building on Debian

Debian does not export /usr/bin in PATH for users like every other
distro so builds fail to find mkfs and parted.

scripts/build.sh

@@ -2,6 +2,9 @@
 
 set -e
 
+# TODO: Move this to build scripts that require it on Debian
+PATH="$PATH:/usr/sbin"
+
 if [ -z "$1" ]
 then
   echo "$0 <model>" >&2

scripts/pxestick.sh

@@ -2,6 +2,8 @@
 
 set -e
 
+PATH="$PATH:/usr/sbin"
+
 rm -rf build/pxestick
 mkdir -p build/pxestick