trusted-firmware-a.mk: Remove spaces from BUILD_STRING var
Those spaces are escaped later and BUILD_STRING looks like: "OpenWrt\ ve43514ab-4\ (mt7986-snand)" which is later used as a const in some c-code which generates compile error: :1:144: error: unknown escape sequence: '\040' [-Werror]
If one knows how to fix this issue keeping spaces I'd appreciated :)