freerdp: pass appropriate ARM ABI flags

In the line of commit 53069378 ("package/freerdp: do not use Neon
extensions when not available") done by Yann E. Morin, freerdp also
passes an explicit -mfloat-abi= flag, and defaults to softfp. This
obviously breaks badly when building an EABIhf system.

This commit therefore fixes freerdp.mk to pass the appropriate
ARM_FP_ABI value to freerdp's build system.

Fixes:

  http://autobuild.buildroot.org/results/6ca/6ca9de1a11c675533baa68f7a6bf7b6af7cb4345/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>