boost: force the target ABI for ARM architecture

boost: force the target ABI for ARM architecture

Just like was done in commit b37641c95be79fcb76cbc7042c4cf21a2c64d0f6
("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>