package/boost: context module needs ARM instructions on ARM

package/boost: context module needs ARM instructions on ARM

The boost-context module contains ARM assembly code, which uses ARM
instructions. Therefore, on Thumb-2 only platforms (such as Cortex-M),
the following build error occurs:

libs/context/src/asm/make_arm_aapcs_elf_gas.S: Assembler messages:
libs/context/src/asm/make_arm_aapcs_elf_gas.S:33: Error: unshifted register required -- `bic a1,a1,#15'
libs/context/src/asm/make_arm_aapcs_elf_gas.S:42: Error: immediate value out of range

To avoid this error, we make the boost-context module depend on
BR2_ARM_CPU_HAS_ARM.

Fixes:

  http://autobuild.buildroot.net/results/8f6770b59a343b9f710e9363b43227ee9f026660

Signed-off-by: Romain Naour <romain.naour@gmail.com>
[Thomas: use BR2_ARM_CPU_HAS_ARM instead of a dependency on
!BR2_ARM_CPU_ARMV7M.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>