build: permit multiple linker scripts

build: permit multiple linker scripts

This change allows platforms to provide more than one linker script to
any image utilizing the `MAKE_BL` build system macro.

This is already done by some MediaTek platforms via the
`EXTRA_LINKERFILE` build system variable, which has now been removed.

In its place, additional linker scripts may be added to the
`<IMAGE>_LINKER_SCRIPT_SOURCES` variable.

BREAKING-CHANGE: The `EXTRA_LINKERFILE` build system variable has been
 replaced with the `<IMAGE>_LINKER_SCRIPT_SOURCES` variable. See the
 commit message for more information.

Change-Id: I3f0b69200d6a4841fd158cd09344ce9e67047271
Signed-off-by: Chris Kay <chris.kay@arm.com>