qt5: support buildroot being located in /usr

qt5 performs sed replacements of /usr to $(STAGING_DIR)/usr in .la files.
However, if buildroot itself is located in /usr, then STAGING_DIR also
starts with /usr, and naive replacements of '/usr' can result in paths of
the form:
    /usr/buildroot/.../sysroot/usr/buildroot/.../sysroot/...

This patch makes the replacements more careful, by first replacing the known
paths BASE_DIR and STAGING_DIR to tokens, then performing the actual
replacement, and finally replacing the tokens with the corresponding paths.

Partially fixes bug #5750 (https://bugs.busybox.net/show_bug.cgi?id=5750

)

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>