Build: strip trailing slashes from directory paths more simply
Build: strip trailing slashes from directory paths more simply

Append . then strip /. seems clumsy.  Just use $(patsubst %/,%, ).

Signed-off-by: Masahiro Yamada <378b411a8a63ecc7605ec4272234862d7781c6ae@socionext.com>

sbranden/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,achingupta/arm-trusted-firmware,achingupta/arm-trusted-firmware,sbranden/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public