Fix rebootstrap and cross-toolchain-base (Closes: #946396):

* Fix rebootstrap and cross-toolchain-base (Closes: #946396):
  - debian/control.in/libc: replace libcrypt1 by ${libcrypt:Depends} and
    libcrypt1-dev by ${libcrypt-dev:Depends}.
  - debian/rules.d/debhelper.mk: add libcrypt:Depends=libcrypt1 and
    libcrypt-dev:Depends=libcrypt1-dev to substvars unless build with stage1
    or stage2 profiles.