toolchain: prepare for more than two alternatives

Lay down the path to add more than two toolchain kinds:
- check the type of toolchain as:
    ifeq (toolchain_buildroot,y)
        blabla buildroot-specific
    else ifeq (toolchain_external,y)
        blabla external-specific
    endif

- prefer using positive checks, a-la:
    ifeq (foo,y)
  instead of:
    ifneq (bar,y)
  (where foo and bar are mutually exclusive)

- have the toolchain_buildroot case always appear first

- gettext is handled differently, because we want to add an option
  only if not using the buildroot toolchain, hence we use ifneq.

Signed-off-by: Yann E. MORIN <yann.morin.1998@anciens.enib.fr>
Acked-By: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>