boost: force the target ABI for ARM architecture

Just like was done in commit b37641c9
("boost: Force the target ABI for MIPS architecture") by Vicente for
the MIPS architecture, this commit also forces the ABI passed to Boost
build system to "aapcs" for the ARM architecture.

Since we now have three cases to handle, an intermediate variable
called BOOST_ABI is introduced.

Fixes:

  http://autobuild.buildroot.org/results/5ae/5aeb3a9f067faf6687051643bf49a0b619cb4c3b/



Cc: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>