Support secondary CPU ABI in build.prop

BoardConfig.mk typically defines TARGET_CPU_ABI to the name of the
native machine code CPU ABI supported by the target device. For example,
existing devices today use the value 'armeabi' corresponding to an
ARMv5TE instruction set with soft-float implementation.

This patch allows this file to also define TARGET_CPU_ABI2 to name
a secondary (minor) CPU ABI also supported by the device. This is useful
when the main ABI is ARMv7-A (identified as 'armeabi-v7a') which also
supports ARMv5TE. Such devices should have TARGET_CPU_ABI defined to
'armeabi-v7a' and TARGET_CPU_ABI2 defined to 'armeabi'.

TARGET_CPU_ABI2 will be translated into the ro.product.cpu.abi2 property
in build.prop. This value will be used by the PackageManager to handle
"fat-binaries" generated with the NDK.