build: Makefile: remove bashism
build: Makefile: remove bashism

Signed-off-by: Alon Bar-Lev <bb308b7fcc6eb1364576b94f9dfcd9006c6c579d@gmail.com>

ARMmbed/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls