toolchain-external: update config options after EABIhf introduction

The introduction of the EABIhf ABI requires a few updates to the
configuration options for external toolchains, in order to ensure that
the user doesn't do any invalid selection. In detail:

 * The Linaro ARM toolchains now depend on BR2_ARM_EABIHF, because
   that's the ABI they use, and it is incompatible with EABI. The
   comment about the availability of Linaro toolchains is updated to
   inform users selecting EABI that they should select EABIhf if they
   want to see Linaro toolchains.

 * The Sourcery CodeBench toolchains now depend on BR2_ARM_EABI,
   because that's the ABI they use. A comment is added to inform users
   that have selected EABIhf that Sourcery CodeBench are only
   available when EABI is used.

 * The Arago toolchains now depend on BR2_ARM_EABI, because that's the
   ABI they use. The description of the ARMv7 Arago toolchain is also
   slightly improved.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>