Cosmetic fixes for build output

By default, the OP-TEE build shows abbreviated output similar to the Linux
kernel. This commit fixes a number of deviations/inconsistencies.

- Always print a command in uppercase, followed by the target file.
- Make sure commands and file paths are nicely aligned, with two leading spaces
to make command output/errors better stand out.
- Remove some duplicate slashes in file paths. We use the following rule:
variables that hold directory names such as $(arch-dir) or $(platform-dir)
always have a trailing slash. The 'subdirs' and 'incdirs' variables used in
sub-makefiles are an exception because doing otherwise would needlessly
clutter the sub-makefiles.