The bash-builtin 'pwd' command uses the 'st_dev' and 'st_ino' members of
the 'stat' struct to compare the path from the 'PWD' environment variable
with the path returned by 'getcwd()'. These members don't get set
correctly in Noux and therefore the 'pwd' command sometimes returns wrong
results when building Genode in Noux. With this patch the 'CURDIR' make
variable gets used instead of calling 'pwd'.
Fixes#454.
